Personally, I think it should be sealed.
Did you end up leaving it sealed? If so, how does it work? Mine has lots of gap and everything leaks like a sieve. In really cold weather the cab of my Burb gets too hot while the defrost doesn't get enough heat because air just bleeds all over. However...
I came across this thread looking for troubleshooting info for something else. I have wiring diagrams for 1990. The attached pictures are for R/V style and P style 1990 Trucks/Suburbans.
Maybe they will help if you haven't figured it out yet.
Happy Fixing!
Jeff