67 Cuda 273 - Extra Circuit for Amp, Seat Heaters

Only way I know to have a circuit energize when the engine is running is to use a oil pressure switch.
The accesory port at one end of the fuse box is nearly worthless in the pre 1970 builds.
They have a less than adiquate wire from the ignition switch going to a 3 leg bus bar there. It already has the heater blower and the stock radio on 2 of the 3.
The accesory port is hot in run though so it could be used for light duty like signaling a relay.