1985 528e - Hot and cold air (ac) at the same time

I have a 1985 528e with AC that works great. But the problem is that I still get hot air coming out of the air vents on my feet and on the vents above the dash. I think my kids said they were getting hot air somewhere in the backseat, too. I've searched around the net to see if anyone else has had this problem but didn't find anything that was exactly the same - only similar problems. From what I gather, this may be a problem with the heater control value. If so, would one of the rebuild kits likely solve the problem? Where is it located on this vehicle? I think I read it was on the firewall, but down low, or something like that. Any help is greatly appreciated. Thanks.