I am having similar, non consistent issue with the back end lights. I am honing in on the issue. I do think it is ultimately going to Reside in the turn signal switch. I have a new switch from She-mar (I have a tilt/telescope column), but I am going to start by reconditioning the existing connector down at the base of the column. Seems like if I give that connector a good wiggle, the rear lights come back to life…
my main issue is intermittent driver side break and blinker go out. My latest wiggle actually brought the dash blinker indicator back to life! I thought those ideas year ago.
I hope FOR YOUR SAKE THAT YOU HAVEN'T OPENED THAT SHE-MAR SWITCH until you've thoroughly EXHAUSTED EVERY OTHER CONTINGENCY. It may well be a connector issue, especially if the old connector has been oxidized, or the wires leading into have been stressed.
Can you make NEW connections which will interface the old connector RELIABLY? If so, do that, then see if your problem is solved. You might yet recuperate the price of the She-mar switch, though it gets harder every day of delay.
If you've unpacked the new switch, then try it and see if your problems vanish. They SHOULD, IFF the switch caused them! I suspect otherwise, though to be sure, worn out contacts in ONE part of a circuit will cause heat, oxidation and failure then in another...and another...ad nauseum,... ad infinitum, or at least until the harness goes Rice Crispies on you.
Now, have you tried hot-wiring each circuit lead from your steering column? THAT SHOULD light up the relevant lights downline from your connector. If NOT, then start tracing that bundle of wires back toward the trunk to the next unpluggable connectors; likely on the driver side, just behind the back seat in the forward area of the trunk, below the left speaker assembly or cutout.
Use a continuity tester from the switch to this connector, one conductor at a time. A test light here serves well.
Bring your hotwire back, and test each conductor to the rear lights at this point too. Any malfunction from this point is in the trunk wiring itself. Grounding issues become HEAVILY PROBABLE.
I pray your weather doesn't resemble mine down here, unless you have a controlled environment to work in.
Happy "rat-chasing" as we used to call such work in the electrician's apprenticeship I misspent several years in SO BADLY! At least I did learn how to use trade grade tools and methods though, in addition to evil vices.