I used it for that over a year with all three and it was ok. And then the driver's side brake light started staying on all the time on the trailer. The only way to get it to go off was unplugging the trailer. I removed the t-1 harness the brake and turn signals worked without the t-1 harness. This spring I roll the trailer out plug it up and nada. None of the lights work? Fuse's and relays are all new but I have no trailer lights at all. Brand new trailer light kit grounded at each light and on the trailer ball hitch. I ordered a oem t-1 harness instead of the aftermarket one. Maybe that will fix the problem I hope.This is correct, basically, plugging in the second relay on an A/T will not harm anything, it just doesn't do anything. My Tekonsha kit came w/ three and I plugged 'em all on, that way I don't have to guess if I left out the correct socket. Works just fine.