I'm currently working on doing this mod to my stereo. It's the premium sound version with all buttons-no knobs and the blaupunkt tape deck. I've had it apart a couple times.
This is the schematic I've found the most helpful. There's info for both the standard delco and premium decks here. Although I'm not planning on using the relay or the double pole switch. Instead I got an 5 pin audio jack at radioshack with a built in bypass. Only 1 switch required I believe.http://www.pontiacbonnevill...nics/topic30441.html