This might apply to 1990-1995 Bonneville SSE and SSEi'* with the 4-wires connectors at the rear 6x9'* -if someone with a 1990-1991 SSE and someone with a 1992-1995 SSE/SSEi wouldn't mind verifying this info I'm sure it will help many.
Well, when I pulled my blown rear speakers last night from my 1998 SSE with the BOSE Dynamic Equalization I found something interesting. I had looked for speaker adapters for the BOSE speaker connectors back when I was looking to replace the 6x9'* in my 1999 SSEi and was told there was no such adapter.
I can now say there is a wiring adapter to hook up to aftermarket speakers, and you already have it. Apparently Pontiac had one made to hook up to the BOSE speakers.
Here'* what I found: