Unfortunately, it wasn't that easy. The harness idea came about because the dealer (Keeler Mercedes, Latham, NY) sent my son a video of a mechanic poking a stick in the trunk near the hing and making the lights work.

I think this is something different now. I pulled the trunk panels and wiggled everything I could find and found nothing. I tested one of the taillights with a PowerProbe and couldn't get the lower brake light bar to light.

Three of the five brake lights don't work. I think all of them have moisture in them.
It doesn't make sense to me that three $300 taillight units would be bad at the same time. I think I like old fashion bulbs better!

I did find a factory manual download for $15. 7,650 pages! and it doesn't have any of the supplemental pages.

Looking at the the wiring diagram, it looks like everything runs from a SAM module in the trunk. Researching this shows that the connection can corrode, but wiggling the plug had no effect. But the SAM module seamed really hot to the touch. I can't imaging it drawing enough power for LED taillights to get hot

So I guess I'm out of my depth with this thing.
My son is calling Keeler and of course it is out of warranty this week

Thanks for any advise.