All wiring notes are referenced to the 1964 Service Manual—one of many found at: http://www.jholst.net/64-service-manual/electrical.pdf Pages 8-95, 8-98 & 8-99.
You are probably aware of these points but my analysis and speculation shows the following:
Only one wire is always “hot” or energized going to the rear of the car. It is an 18 ga Pink wire M1/M1A and it goes from the 15-Amp ”TAIL/STOP/DOME” fuse on the fuse block to the pillar lamps in a coupe or the “pocket panel ” lamps in a convertible and the trunk lamp.
The two tail lamps and the license plate lamp are fed from the R terminal on the headlight switch through an 18 Ga Black wire L7. Both of these wires and five others are coupled at a black plastic connector (rear body wiring) behind the LH kick panel and then run in a fabric-wrapped bundle to the rear of the car under the sill plate.
You could pull the ”TAIL/STOP/DOME” fuse which would de-energize the Pink M1/M1A wire just to confirm the stray juice is coming from that feed. If pulling the fuse does not stop the problem, the stray source becomes harder to predict and find but would come from a cross-connection under the dash or from some other +12V source. Trace the path and condition of the Black L7 wire from the headlight switch connector to the rear body wiring connector. They are only a couple of feet apart but hard to see and trace.
Faulty headlamp switch. Unlikely, especially since you replaced the switch and that the ten or so wires that are connected to it are arranged in a large black plastic connector—eliminating the possibility of putting a connector on the wrong spade. Disconnecting the large black connector from the body of the switch should confirm whether the switch is innocent or guilty. Remember, these lights would come on if the light switch was only partially pulled out—to the “Park Light” position. The knob that operates this switch is connected to a long slender triangular spear that slides in and is locked in place . It is released by pressing a release button on the bottom of the switch. Make sure all is engaged here and that the parking lights, headlights and courtesy lights are coming on properly by pulling, pushing or twisting the switch. If the new switch and old knob and spear are not compatible, the park light internal switch may be unintentionally energized.
Shorting across from an “always hot” wire to the taillight wire: Could be in the dash area or in the rear of the car. Disconnect the seven-wire/eight slot connector found behind the kick panel. Connect the battery and check the Voltage to ground, if any, at the Black wire spade on the connector end of the fabric-wrapped bunch of wires from under the dash. If there is no Voltage, the problem is from the connector to the point on the rear loom where the trunk light wire (M11-Pink) comes out of the loom and goes to the trunk light bulb socket.
If in the rear, it is always possible the wiring loom got pinched during restoration. This portion of the loom is not too complex but well-hidden and protected under the sill. Since something changed after working well I suspect a screw or metal plate might have contacted the rear wiring loom
It is worth noting that the Pink and Black wires are side-by-side in the rear body wiring connector. Examination for a faulty connector or a stray piece of wire or metal might prove productive.
Going way outside the box, please note if the stoplights/bright filaments are also coming on and stay on. If so, they could be feeding the tails thru a bad lamp socket.
One more thing: If your car is a convertible, there are a pair of wires running from the top switch to the pump that share the same trench as the lighting wire loom. IF the top switch is inadvertently switched on, one of these wires would become a source of power to the rear. One would expect to hear the pump running in that instance, though.
Stick with it. Something major may be wrong and might pose a threat to your life, car, garage, resto job & etc.
Posted by: "Rich Barber" <c300@xxxxxxx>
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