My AC compressor is engaging when I push my heat button "in". Per the FSM the compressor clutch should be off. I supposed there is 12 volts going to the compressor when it shouldn't. Any idea where I can look for this problem?

Thanks!

Is it in the defrost mode?

No it is in the heat mode. When I hit the defrost button I don't get much air at all at the glass but get the same on the floor as heat mode.
I restored the car including rebuilding the heater box. I thought I had the vacuum lines all back in the right places? I did read that the linkage on the defroster duct could need to be adjusted to get the correct air flow. Prior to restoration I never ran the heat because of a bad heater core. But did run the AC and it worked well.

I rebuilt one for a 69 Charger and bench tested all of my servos. I had one that was failed on almost every spare box I had. You need to test them before you install those. They are 50 years old and have a rubber diaphragm. You may not have the correct plastic switch on it as well. A 69 B body has a different one with a pigtail plug on it. All the ones you can still buy are direct plug in. Not sure they interchange.

What plastic switch are you referring to? There is a pigtail that comes out of the compressor with a plastic plug. I did replace that wire and plug, and the wire from the firewall to the compressor is new. I did not test the compressor prior to installing it.

Even though most of the 5 button A/C switches look the same they do not function the same. Your car calls for part #3431020. If you installed part #3502126, pushing the heat button activates the compressor. That switch is for a C body. The pics below show that they both look the same. Many Ebay sellers list them as fitting all applications, which is wrong.

Thanks for the info, but I never took out the original. Still works fine, unless this is the culprit.

Look at your original unit. These went out on a regular basis and were changed often. As I said, your oem dated one should have a pigtail soldered on to those three prongs in the middle of the switch. Look for the part no and date code and post it here.

Ok, for now I have disconnected the power feed to the compressor. Heat works well. Will post up the numbers