The pressure switch that disabled the A/C system on low pressure was not introduced until after 1969 for Plymouth and Dodge; don't know about Chrysler. Plymouth diagrams show dark blue colored wire from the firewall to the compressor clutch connector. The wire after the connector to the clutch is usually light green. But you need a 69 300 wiring diagram and FSM.
When the heater is off, no coolant is supposed to be flowing through the heater core, and the doors inside the heater box should be closed. Maybe the vacuum hose/vacuum motor system and vacuum switch that control the heater box doors isn't working properly. If the heater core was hot, sounds like the coolant valve wasn't closed. Start troubleshooting with the valve, then the doors. If your dash controls are shot, that could explain both the heater hot in off and clutch wire not hot when A/C on.
Yes, if the compressor cratered internally, the entire system is contaminated with metal debris that is usually visible in a black coating on the expansion valve and at pipe ends. All piping/hoses, the condenser, and the evaporator must be flushed with flush fluid and shop air (about 100 psi), NOT an aerosol flush can. For an example of a flush tool, see:
http://www.northerntool.com/shop/tools/N...g%2Bflush%2BkitReplace the contaminated accumulator/drier and the expansion valve. Flushing will remove all refrigerant oil, so the oil must be replaced with refrigerant compatible oil, either R12 oil or R134a oil per the FSM, a few ounces each into the new compressor, new accumulator, and each heat exchanger for the specified total amount.
This photo shows a Plymouth dark blue clutch wire running to the front of the valve cover terminating at a connector and a light green clutch wire after.
