Can I take a cheap RC car, take it apart, and hook the wires that are supposed to go to the motors as inputs? I assume I'll need to make sure they have a low enough voltage, or use some resistors to lower it. But is there any other reason it wouldn't work? Seems like a cheap way to get a remote control working.
Can I take a cheap RC car
Yes, but pay for it on your way out.
take it apart
It's yours. Do what you like with it.
and hook the wires that are supposed to go to the motors as inputs?
Hook them where?
Seems like a cheap way to get a remote control working.
Remote control of what?
I don't see why not. I usually think of going the other way with the arduino hooked up to the remote but it's a cute idea to go the other way.
As you say look at the voltage but it's well worth trying. Some of these things run their motors in both directions, presumably by reversing polarity - watch out for that! I imagine also that the motor drives will be pulsed but you can probably still pick something up.
Don't forget to post results.
If the motor control is pulsed, you can get ICs to change that into a nice steady voltage.