The way I understand it is, there is a constant 12volts being supplied to the TT from the TV when driving down the road. In my case the constant 12 volt relay was not installed.The gain control worked, I tested it, it locked up the brakes on the TT the first time I tried it.
There were other issues that did not, seem related, because of my inexperience. Such as, the battery would not recharge when the truck was hooked up. I also unhooked the battery on the TT, took it completely out of the circuit. Hooked up to the truck and then tried to retract the slide, nothing moved. So, I should have known. :o