Heat/AC Control Replacements Keep Going Bad!

My 96 Buick Regal Heat/AC control had illuminating lights that burned out. I had to unsolder and solder in replacements that I got at Radio Shack. After a while I noticed that the control unit wasn’t working quite right; in non dual control mode where the driver’s temp control rheostat controls driver and passenger air duct temp, passenger air was cool when rheostat was turned full warm, driver’s duct temp hot, as it should be. In dual mode, where driver and passenger duct air temps are controlled independently, unit functioned properly. So I replaced it with a junkyard unit that worked properly in every way, except several lights were burned out. A dealer wanted about $400 for the whole unit when I checked into having the lamps replaced in the original one almost 10 years ago, if still available it would cost more now, no doubt. So it was worth it to buy a used one around $35 and change lamps myself again. I accidentally reversed the polarity of 12 volts I applied to the lamp circuit to check my replacement job before reassembling the unit and found the same problem happened again like in the original unit, cool passenger air in non dual control mode. I blamed myself for reversing polarity, figuring this was the problem, I probably reversed polarity the first time I changed burned out lamps also. So, a while later I again started wanting the control to work properly, found another used one, of course it also had a few burned out lamps, but it worked perfectly, I replaced bad lamps in this one also, and made sure I did not check lamp function with voltage applied to lamp circuit this time, and to my suprise and frustration, I had the same cool passenger air non dual control problem again! Does anyone know how just replacing lamps with ones that fit and are 12volt lamps and low milliamp current draw can keep causing this problem? I don’t think I did anything wrong or broke anything in doing this lamp replacement job! Sorry about the length of this post, but I didn’t know how to make this long story short!