Reliance brake controller E2 code

With only 12 volts present in the braking system, rain water doesn't conduct electricity well enough to cause a short circuit. (It's not a perfect electrical insulator, but it's a VERY poor conductor).

So, probably best not to completely rule out some other cause of the short.

Inspect the wiring, looking for any exposed connections that might be occasionally touching the trailer chassis, and for frayed insulation on the wires.