Good info JonJon. All the returns are low and wide open. Unfortunately I have no access to the duct work that goes beyond the garage. I guess I could sledge a few cinder blocks out to take a look... but I think I'll get a pro to do it so that if something happens along the way I have someone other than myself to blame.
I'll insulate that room when it warms up a bit though. That and replace the old windows.