Sounds like you have two practical choices. Either go to a junk yard and find a similar radio and make the swap, or buy a new unit and install it. Trying to troubleshoot the switch is difficult, and finding parts to replace it wouldn't be worth the trouble.

-Mark- '99 Mystique LS, totally original (including the original water pump and the premium stereo)! Bought new in December 1998 for $21,000. 208,000 miles