I have an 89 s5 with a broken power window switch on the drivers side. I know that a switch from an s4 or s5 will work as a replacement, but I was wondering if the fd switch could possibly work as well or if anyone has tried this?

 

Sure, the FD switch could be made to work- just like the MPV switch can- but I don't think the shape is right.
Seems to me that the baseplate on the FD has an arc to it while the FC is flat.
Besides, the FD is about the last car you'd want to use as a donor...their stuff is pure crap.

 

Power window relay mod!

http://www.ausrotary.com/viewtopic.php?t=123079

The idea is that your window switches probably will work, but it's not enough to pass all the current to the motor. It doesn't require near as much current to operate the coil of a relay, and you can get full current to the motors with the relay contacts. Best part is that the relays are cheap and will ALWAYS be replaceable, vs trying to use a 25 year old proprietary design.

 

Ur gonna be cleaning/swapping out those switches on a regular basis. Remove the power switches and switch to manual roll-ups.