I checked the instructions for the Roadmaster InvisiBrake Supplemental Braking System part # RM-8700, in order to set the brake pressure you must first pull the breakaway pin and then pull out on the knob. Then, rotate to the designed psi, reengage the knob, and reinsert the breakaway pin. I have attached a photo below with these instructions. If the brake pressure still fails to set, there may be a hardware issue and a warranty claim with the manufacturer would be needed. view full answer...
It sounds like you have a bad/failed connection on your installation of the Tow Ready Tail Light Isolating Diode System with 4-Pole Connector, # TR118821. I suggest that you first check the ground connection and ensure that it is made to a clean bare metal surface that is free of corrosion and verify all connector pins are properly seated and not damaged. Then work back from one end of circuit to the other performing a continuity check and looking for any damaged wires. view full answer...
If I had to guess as to what happened I would say that they connected the P2 brake controller # 90885 red wire to the wrong wire on your 2018 Jeep Wrangler Unlimited (JK). It is a difficult wire to get to for sure on most Wranglers. They may have used a different wire that carried more than just the brake light signal thus causing an overload condition.
Take a look at the video I have linked showing installation of a brake controller on a Jeep just like yours. The controller is different... view full answer...
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