While it sounds simple, sometime it isn't that simple particularly if you want to control
things when the ignition is off.
For example suppose you want to control the headlights even when the ignition is off.
In some cars the steering column switch turns on power or grounds a signal that controls a high current relay
but the high power circuit or the power to the high power relay is not active until the key is turned on.
In this situation, if you tapped into the column switch for the headlights, it wouldn't be good enough
to turn the lights on if the ignition is off.
Things like windows can be particularly tricky as those circuits often are disabled when the key is off.
In order to control those you may have to energize the window control module (if there is one) or
control the motors directly. And if you control the motors directly you must make sure that
you don't create a situation where your circut and the cars circuit can potentially "fight" over controlling
the motor. If you do, you will blow fuses or worse burn up wires.
Power locks can be a bit tricky as well. Some cars have lock control modules and some don't.
(most newer cars have modules)
Some cars don't have lock/unlock switches.
In those, the contacts are built into the lock solenoid.
You can often tap into the wires used, but some locks don't use simple on/off connections
or + or - signals for "lock" or "unlock". Some use pulses, or double pulses, and some use signal levels that invert
depending on the current state of the door lock.
It all depends on what you want to do, but these are some of the things
you may have to deal with, and having a wiring digram can make things so much easier
since you see which wire you need to tap and often what type of signal is required.
Another thing to consider, it is that often even if there is a switch, it may be considerably
easier to tap into the very same wires that go to the switch somewhere else.
This is often the case for switches/signals/wires in the steering column or inside the door.
The wiring diagram will show you the colors of the wires and usually where they run inside
the car.
--- bill