I know nothing about the set up on the Mercedes, that said, Fords used to come with a wiring harness, that if one wanted to tow, one had to get under the dash and connect the harness to the harness which ran from the dash to the back of the Ford vehicle. This was true with the older Explorers at least. The easy way to test your TV circuits is with a 12v
test light, ground to a metal surface, stick the prong into the 7 way plug and see if it lights up. If it does test it on all circuits with someone sitting in the drivers seat working the brakes and turn signals as you instruct. If all works then it's either the connection or something in the trailrer.