Barry, assuming the fuse doesn't blow when the AS is disconnected, you've got a short to ground somewhere in your trailer.
I think I would start trouble shooting by looking over the 7 pin. Then by pulling the bulbs and looking things over for something obvious with bulb or socket. Then, with the bulbs out, use an Ohm meter and check from your male 7 pin plug brake light terminals to ground (the trailer frame or skin, or the ground terminal on the connector). You should not (my best guess) be able to read continuity to ground. If you do, that's the circuit that needs more investigation.
Hope that helps.