Your problem started when you installed LED TBL because that eliminated the last ground path that was needed. If you had Oznium and stock tails everything would be fine, because ground would pass through the tail light bulbs. But with all 3 LED brake signals you need a dummy bulb just like i suggested:

 

OOOOhhh!!! Ok that makes sense. Wow, pretty amazing how one bulb could be the difference. Thanks Reactor

 

Reactor- Do you have any explanation to the fact that my trailer lighting harness still causes problems even after wiring my Oznium TBL to the OEM TBL? Seems like it would work now considering my trailer lighting harness has always worked up until the Oznium TBL (even with my LED tails).

 

Sure, here is my theory. If you have a voltmeter you can prove me right or wrong.
The trailer harness gives you the problem because it internally connects 2 circuits that normally are completely separate on xB: turn signal and brake light. Since the trailer uses a single bulb for both functions the added harness merges brake and turn signal wires together. That feeds the positive voltage from the turn signal into the stop light wire. The car's computer monitors the stop light wire so it knows when you apply the brakes and it needs to turn off the Torque Converter Clutch. That's why it was doing that when you used your blinkers.
Now all that would not be a problem when using regular bulbs in the tails and stops because bulbs have enough resistance to drop the unwanted voltage below the level that ECU can detect. Let's say it wants to see 7V when you push the brake. With the trailer harness and all stock bulbs it would leak may be 3V into the brake wire when you use the blinker. With LED tails that may go up to 5V - still not a problem. But with Oznium TBL that voltage went past the "trigger level" of 7V and that's when you started having issues.
Rostra did exactly the same thing. It feeds small (may be 5V) positive voltage into the stop wire. That's why your TBL was at 50% brightness when you activated Cruise. So it's the different add on device that's creating the same issue: unwanted positive voltage into the stop light wire. Trailer harness leaks few volts, Rostra adds another few and without the regular bulbs acting like a ballast to drop that voltage below the safe limit your ECU thinks you are pushing the brake pedal.
You can try adding another dummy bulb in parallel, may be even 2 and see if that lets you run both Rostra and trailer - it may work even though i feel that the trailer harness is basically wrong for the car. Aftermarket TBL, aftermarket LED tails, cruise control and trailer harness: that's a lot of things sharing a stock circuit so it may take some effort to do everything safe

 

That makes so much sense it ain't even funny. Yeah, I am overloading those circuit's like nobody's business. That theory is probably correct. And if that theory is correct, then I will still probably be unable to run the trailer harness WHILE removing the Oznium altogether because that damn cruise is now leaking voltage too! and that voltage is over the limit (with the LED tails of course). Is that correct?

 

Yes, that may be asking too much from the stock circuit to handle. Especially since the design of the trailer harness is so not LED friendly.
It's possible to redesign the way Rostra and the trailer connect to the car so that everything works fine with your LEDs, you just need a good electrician to spend time with it.