I know just enough about electronics that I think it's a wonder I haven't died from touching both ends of a flashlight battery, but here is my problem and I'm hoping someone out there can guide me.
I'm not a mechanic, nor do I play one on TV, but I did fix my wife A/C blower motor on her 93 Taurus this morning (some piece of foam rubber got in the squirrel cage and made it vibrate - damn, but they sure do put the screws for the cover in odd and difficult to reach spots!). I try to do what work I can on my autos, so I am going to try to help you a bit here...
I am restoring a 1957 vintage Chrysler dealer-installed air conditioning system in my 1957 Chrysler. The evaporator unit that mounts on the transmission hump has a little fan on it that has a two speed motor. But apparently there must be a short in the motor that causes it to run at high speed no matter which speed you switch it to. When you energize either one of the speed wires the motor runs, and the other speed wire also tests energized. Even if both speeds were working fine, you'd really want one somewhere between the two, so I want to go for a better solution than fixing what may be wrong in the motor.
Personally, I would look into seeing what is wrong with the motor; why it isn't switching speeds correctly. Even if you do put in your modification for PWM as you mentioned (though depending on whether this is a show-car or such, you might want to think about this), you'd be doing so on a motor that has a problem which could come back to haunt you at any time. Fix that issue first, then work on the PWM control. Likely it is either a burned-out resistor pack (basically resistors of different values switched in to control current - the resistors will be high-wattage wire-wound with sand/ceramic, or possibly heavy duty heating element wire wound on some kind of heat resistant core), or if the 57 is older than resistor packs (like I said, I'm not a mechanic!), then it might be some kind of weird switching arrangement on a series/shunt wound DC motor, to switch portions of the coil in/out to change the strength of the magnetic fields (and thus the speed of the motor). I suppose that there could also be other methods as well.
You might want to get in contact with a local or nearby vintage automobile club in your area; they could put you in contact with someone to help you get this diagnosed and fixed. I would also suggest looking into finding (it will be difficult) a repair manual for that year/make/model - look into old back issues of Popular Science magazine (on Google Books) to find advertisements on the auto-repair manuals (I am not sure Haynes or Chilton were doing it back then, but I have read that decade of PopSci, and there were other publishers of such repair manuals - so you might look into those; check via Amazon, Alibris, and Abe Books, among other online book sellers - oh, also Ebay). You might also find in the pages of PopSci some articles or "tips-n-tricks" type blurbs on fixing your blower motor; A/C units were a fairly new thing of the time period, and I do remember seeing a few articles about them in the magazine (on how to install third-party units, how they worked, etc). Lastly - Popular Mechanics magazine is also in Google Books, so check those out as well.
My plan is to put what I understand is called a PWM in one of the speed wires and disconnect the other one. That way I will have infinite control of the range of speed for the fan. Now, all the info I have dredged up on PWMs says they are made to work on brushless motors. I don't know if brushless motors were even born in 1957 yet, but this certainly isn't one. And for the life of me I can't figure out if a PWM is supposed to work on a brush type motor as well.
First off, understand that PWM is a control method (not just for motors) - it stands for "Pulse-Width Modulation"; basically it's a method of switching current to a device (whether a motor, an LED, a lamp, a speaker, or something else) at a set frequency, while varying the time the current is "off" vs the time it is "on" (if you are familiar with welding, it is similar to a "duty cycle" - the amount of time you can use the welder before you have to let it "rest" - though a duty-cycle is a much longer and drawn out form of "PWM").
What you were probably looking at (and here's another thing I have noticed - in auto mechanics, they use one terminology to explain another - and confuse the hell outta people who know both) were "motor controllers", which use PWM to control the speed of a motor.
Now - your motor is a brushed motor (whether it is a permanent magnet motor, or some form of shunt/series/combo wound - that's a wholly different question); while brushless motors did exist (ie, 3-phase AC motors, and/or shaded pole AC motors) in 1957, they were not likely used (as you would had to convert the DC of the battery to AC - they had to electro-mechanically do this just to get radio to work, because of the tubes in the radios needing the higher voltages). Likely the motor is either a permanent magnet motor, or possibly a shunt/series wound.
You will want a motor controller with PWM (and it doesn't need reverse capability) for a brushed DC motor, at the voltage the motor uses (you are claiming 12 volts, and you know your car - but 6 volts was pretty common in the period; your car was in the "transition" from 6 volt to 12 volt systems - heh, reading PopSci, they were also thinking about using hydraulics everywhere for power accessories!), for the current it needs.
This 12V motor is drawing about 8 amps on startup and 4 amps running.
Something tells me this isn't a permanent magnet DC motor, given the current draw - probably shunt or combo (I suppose it could be series wound, but those were prone to "runaway" with light loads, and I think a fan would be considered a "light load" - dunno).
Can I buy just any old PWM of perhaps 10-15 amp capacity and hook it up inline to this motor to control the speed?
Such a motor controller would probably work OK, though I think on the low end I would go with 12 amps (2 amps extra doesn't seem like enough headroom to me).
Or do I have to use use a resistor type controller for this brush-type motor? What should I look for and where should I buy one ready to plug in?
Nothing that you buy will just "plug in" - if you want to install something like this into your system, you are going to have to do some "hacking" here to get it to work. If you wanted to go with a resistor controller (they use them in automobiles for blowers even today on modern cars; they are cheap, relatively robust, but you have to mount them to something to act as a heatsink for the extra heat they produce so they don't burn out), I would look into how modern cars have them hooked up, generally (IIRC) they are simply a couple large high-wattage power resistors with three "taps" - the lowest tap connects direct to the power input from the fuse block (high speed), then next tap is across the first resistor (medium speed), and the last tap is across both resistors (low speed); you could add a third or fourth resistor (and taps) to add a few lower speeds if you need them. You would also need a multi-position switch, of course (rated for the current); the resistors would likely be 20-25 watt or higher power resistors, probably 5-15 ohms each - something like that. Get ones with metal heatsinks attached, then mount those to a larger piece of metal or to the firewall of the vehicle (or if you can manage it without damage or modification, to the evaporator of the A/C unit via a mounting screw, so that the A/C carries away the excess heat).
Note that all my numbers above are "guesses" - I don't know much about resistor packs as used on vehicle A/C blower systems, so do a lot of research beforehand if you take this route instead of a motor controller. You might also look into buying a used but working resistor pack for a vehicle with similar blower specs from a pick-ur-part place. They likely have tons of them (and might be able to recommend you one).
Finally - I want to re-iterate to you the importance of getting that motor fixed -first-; like I said, look into local vintage automobile clubs in your area, you might find a person who was a mechanic of the era (heh - maybe you'll find a "Gus Wilson" old-timer who knows exactly what to do!) who can help you out, and get it working right. You might also find somebody who can help you install or set up a motor controller with PWM or a resistor pack, and do it in such a way so-as not to ruin the value of the vehicle (especially if it is a show car).
Good luck, and I hope the above helps you out!