...Many controller companies warn that if you hook up to the wrong polarity, it may destroy the controller.
always test the breakaway with the tow vehicle unplugged electrically from the trailer.
It could also be a dramatic short across the battery
, since the circuit you're trying to build connects the battery to the brakes
through the breakaway switch (on the + side) and the ground connection (on the - side). If the breakaway switch is mis-wired to connect battery + to brake ground side, that would be a short circuit through the ground wiring.
A breakaway switch connecting the battery ground to the brake power (+) side wouldn't do anything when unplugged from the tow vehicle (and wouldn't work), but I can see how that would be bad for the controller.
So it's battery + terminal to the breakaway switch, then switch to brake power side (whichever wire the blue wire from the tow vehicle supplies). Of couse the original question is which wires are these in a Casita
, and I can't help there.
The Tekonsha Instructions
are a good generic guide, found in their Breakaway Systems and Accessories