I know this post is old but was hoping you (or someone else who did this wiring) could help, as I have a very odd problem that sounds similar to what you have seen.
Although my trailer is all LED, I decided to buy an isolation trailer adapter from Hopkins just in case I forgot and tried to connect a trailer with incandescent bulbs to it. On the Tesla, I found the right turn, the left turn and the parking light wires and connected them to the respective "ins" of the adapter (right, left, parking). For power, I tapped into the power lift +12V to run the adapter (plus I found/confirmed a good ground as well).
When I put on the right turn signal, it works fine. When I press the brakes, both lights come on and work fine. When I turn on the parking lights, both trailer lights come on at the expected lower intensity as do the side lights on the trailer. And with the lights on (parking or headlights), all the lights (left, right, parking, brake) work fine too.
But here is the really weird part - with the lights off, if I turn on the left turn signal, it blinks as expected BUT the right parking light (lower intensity) also blinks. If I turn on the parking or headlights, it then works fine. I tried my harness with 2 different trailers that have LEDs - in both cases it does the exact same thing. I am at a loss as to why, with this isolation kit, it wants to flash the right parking light as well as the left turn signal light (which of course is also the brake light on that side). I did 'tap' into the parking light wire on the right side of the Tesla if that makes any difference.
Any idea as to what is wrong?
Thanks.