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My son's 75 has a continuous draw on the battery that kills it real fast. I followed the troubleshooting advice from the Forum and narrowed the problem down to the courtesy lights circuit. Question now is where to look from there?
Also, the driver tail/brake light is not working right. The tail light will come on, but the brake light will not. Checked the bulb and it's OK. Checked the circuit (yellow wire) from the light receptacle to the rear harness and from the harness to the wire loom on the column. Both circuits reading 0 ohms. The passenger side light is working properly.
I just went thru basically the same scenario with my 77 except it was blowing the fuse. Do you have the wiring diagram? On mine the courtesy lights circuit power the clock and the cigar lighter and the factory alarm system. First thing I would do is pull the courtesy light fuse and see for sure it cures your drain problem. The brake light first I would make sure the ground is good. Does the turn signal on that side work?
On the lr brake light. Remove the bulb and check the contact for 12 volts. If its not there then check for 12 volts at the yellow wire at the harmonica connector.
Battery drain. remove all easy items from the courtesy light circuit. remove all the bulbs including the one in the small rear compartment,unplug anything on the circuit then see if the draw is still there. If the draws is gone start putting things back till you find it.