Blown left brake/turn signal fuse (Explorer)

I am towing a 25/18 TM with a 98 Explorer. The Explorer is equipped with the tow package. When we started towing on a recent trip we noticed that we did not have a working left turn signal and brake light on the TM. We changed the bulb and that was not the issue. We checked the fuses in the Explorer and the left turn/brake fuse for the trailer was blown. We replaced it and it was working. When we got to the next stop we noticed the fuse blew again. This went on for the entire trip.

My mechanic feels that it is something in the TM since it doesn't blow the fuse when the TM is not connected.

Does anyone know of a way to diagnose and fix the issue?

Thanks
Wayne