I just purchased a 2007 Ram 1500 that failed state inspection because the license plate lights flash when left turn signal is on.
Symptoms:

Lights off, left turn signal: License plate lights flash.
Lights off, right turn signal: no issues
Lights on, no issues.

I have swapped left and right rear light housings to see if it was the lights themselves, but that didn't resolve anything. Interestingly the Reverse Light and License Plate lights use same socket. When I swapped the sockets, and left the reverse light bulb in, there were no issues. The reverse light bulb however is too big to fit in the license plate housing.....

It's odd that the Reverse Light has less resistance (1.5 Ohms) than the License Plate bulb (4.5 Ohms).
I have started going hand over hand on the wiring harness, but watned to come here to check to see fi anyone has seen this /resolved this before.

Thanks

 

These are also prone to bad FCM modules….the silver box mounted to fuse block….the fcm controls the lights, I had an issue, no low beams or front marker lights, high and everything else worked, but when I turned lights off markers would then come on…..it was my FCM

 

Thanks, I will check on that. Also wondering about the 4 prong plug for the trailer lights, which is quite corroded.

 

That could absolutely do it since it’s bridged into the tail light harness

 

Do you have the correct reverse lamp bulbs?

 

yes?
912/921?

 

Add a ground wire to the offending side tail lamp side and I bet your problem goes away... likely its finding ground through your reverse lamp circuit. Had a similar grounding issue on my 2000 2500, I just used one of those cheap side clamp connectors and ran a wire to the fender.