It's a drivability issue with the oil pressure switch. It prevents the electric choke from heating up until the engine has started. At the least you want an ignition circuit, one that is on only in the "run" position and never any other time. Otherwise you listen to your radio one day waiting for someone and your car won't start.
The manifold heater draws a lot of current, the oil pressure switch triggers a relay somewhere taped into the wiring harness one the firewall (that might help you find the wires). It's like a set of diesel glow plugs, and AMC didn't want it killing your battery if the alternator wasn't running to power it.
The part of the harness that controls the oil pressure switch, manifold heater, and choke heater is a sub-harness that plugs into the main harness somewhere on the passenger side.
Here's a diagram of the circuit.