A few years ago I replaced the heater core in my 93 LX. When I was done with it the only vents working were the defrost. It bothered me but the heat worked. I recently found the problem and it was literally right in front of me, so I thought I would share.
On the drivers side under the hood there is a vacuum block / tee. A small plastic line was brittle and broke. when I reattached it all the vents started working again. I had the same situation on the 86 and the same fix.
On the drivers side under the hood there is a vacuum block / tee. A small plastic line was brittle and broke. when I reattached it all the vents started working again. I had the same situation on the 86 and the same fix.