So my 2004 3500 cummins is having a strange problem where only the rear right turn signal works and I thought it was maybe a bulb but when I hooked up the left tail light to the right plug the blinker worked. I've tried everything in my power to fix it but cant figure it out. All bulbs are good. I have replaced the multifunction switch and that did nothing.

 

Most likely the circuit board in the tail light .

 

i can confirm that its not

 

Check your fuses. I once tried putting aftermarket LED tail lights on my old truck before I sold it, and it made my fuse go bad...

 

If all grounds an wiring are good then being 2004 Ram it might be the BCM or some call it the FMC ,,this is the body control module and controls lights of the truck,,high an low beams ,,signals and everything ,,it can let one high beam not work to low beam not working from side to side and mess with the rear lights as well ,,its common on these trucks,,it attaches to fuse box under hood,,my personal problem was I had passenger side high beam out ,,I was able to unplug the module and clean contacts and I used some small washers as spacers to reinstall , and by shimming it out let my lights work properly again , don't ask me why,,some days it acts up and I just push on the module and it comes back and keeps working,,I believe it wears out area were it plugs into fuse box, the extra washers use to shim it out let mine get better contact ,,
I cant promise its your issue but it is a place to start