I didn't have much luck searching for the answer to this question, but eventually found the correct part number for the retaining clip for the 4th gen pedal assembly to allow you to use the third gen brake switch.

Part number 10200424

I could not use the 3rd gen brake switch with the assembly I had. The 3rd gen switch is normally closed and the 4th gen switch is normally open. OR the reverse, I can't recall. So either use a relay with the 4th gen switch or get the correct retaining clip for about $10 from GM. Part number is above.

If this is posted somewhere else forgive me reposting this, but I spent a whole lot of time rewiring and screwing around under the dash before I found the correct info! Mods please add this to the faq board or wherever it needs to be.

 

All i did was screw the 4th gen switches out of their clips and screw the thirdgen ones in their place. i had more trouble getting around the cable that used to go to my shifter from the steering column. it kept getting in the way

 

Ah. I suppose my problem was I lost the retaining clip somewhere along the way? At any rate, maybe this will help someone at some point.