Road Runner Nest

all new wiring from yearone(except rear taillight harness and steering column), been fine the past 2/3 years no issues, now at night if my headlights are on, no issue. If I switch to high beam my left turn signal stays on. Going to replace the steering harness this winter....

Two harnesses coming from the column. One is the turn signal/hazard light combo switch. The other is the ignition switch. Don't recall which one carries the horn circuit.

Your situation is strange since the headlight circuits don't go into the column. I'll look at the wiring diagrams in the morning for some possible explanation.

When I reassembled my car this past summer I was having intermittent rear signal/brake/hazard lamp issues on one side, and figured my switch went wacky, so I bought a new one. One day before putting the new switch in I was trying to make the problem happen and found that wiggling the wiring where it exits the trunk to the bumper would make the problem come and go. Since I had replaced the harness from the kick panel to the trunk when reassembling, I assumed the bumper harness had a broken wire. WRONG. It was the brand new harness made by M&H.

So now I have a switch I can play around with.

There are two places where the hi-beam circuit and turn signal circuits come into proximity with each other. In the forward lighting harness and the dash harness. You seem to have a cross-connection between the hi-beam and left signal that shouldn't exist. Unless some wiring got displaced, I'd be looking for a short between the circuits due to some worn or dried away insulation. The forward harness is relatively easy to replace. NOT SO with the dash harness.

Any help?

Check the grounding of the turn signal housing to the body. Make a jumper and ground directly to the battery and the other end go straight to the bulb holder of the turn signal. If it eliminates the problem, you will have to back track to find where you are losing the ground. These old Mopars are notorious for having bad grounds. I had a bad ground between the bulb socket and the housing. Had to stake it in a few places with a chisel to get a solid ground.
Ron

Have you figured out where the issue coming from? My buddy's RR is having almost the same intermittent on turn signal when the high beam is on.