I searched a bit. I looked in the FSM for 1990 and didn't see the Bose system detailed, just a four speaker system. 1991 had a Bose system, but no center channel mentioned. So I looked more and found an older thread here:
bose-center-channel-info-t572593.html
It's got some pictures and a bit of wiring described. You can look into the FSM for nearby years and maybe that will have the stereo parts spelled out a bit better. It seems as if the front speakers had an amp built into them (bose system) and that the center channel parasites off of both the front speakers for it's power... likely through a harness. But, that would seem that you have to have the Bose front left and right to make the power for the Bose center... that could be an issue.
When you're done having fun with this, replacing all with aftermarket componets will be fun. I recognize the Rainbow name right away... some very nice, although very expensive, sound quality from those drivers. I remember coveting those and some ribbon tweeters back in the day.