Is it possible to rig a window motor with 3 magnetic switches to locate center, 90 degrees left, and 90 degrees right, similar to a servo?
I have seen a lot of tutorials to build a window motor servo.
But, I do not need precise movement, and I want to use magnetic switches because I believe they are waterproof.
Yes, you can do this. Because such motors can tend to be difficult to disassemble (varies from motor to motor), it is probably best to keep the magnet(s) and switch(es) external to the motor itself (if you can take the motor apart easily, and it has a plastic body so-as not to interfere with the magnetic field, then you might be able to come up with a more compact or integrated system).
The basic idea is (and you can play with this - there may be other arrangements that can result in fewer magnets or switches needed - give it some thought) to put a magnet on the moving part, and as it passes near a switch, it will activate it. Each of these three switches would be connected to an Arduino digital input, which would then be monitored by the Arduino to determine the motor's position.
It would probably be easier, though, to instead have two extreme left/right limit switches set up (with or without monitoring by the Arduino), and a center switch that was monitored by the Arduino; this would reduce the number of digital inputs needed to one ("return to center", in effect). The Arduino would control the h-bridge controlling the motor, but when the end stops were reached, current would be cut off automatically to the motor. You can find out how to connect such switches from this old thread: