Acc circuit on the ignition switch. If you have a properly installed heater kit, the circuit is already tapped. I'd make a proper Packard connector "Y" jumper from the load side of the heater circuit breaker and share the breaker between the heater and your disco light.
I should be finishing my heater install tomorrow or Saturday (then my hands and arms can quit bleeding and heal). If you need pictures of the wiring, ignition switch or circuit breaker, let me know early and I'll try to accomodate.
Whatever you do, don't splice into your harness with Scotch-Loks or any other shortcut methods. Always use the proper military connectors and jumpers.