I think you're meant to have a self-resetting circuit breaker rather than a fuse (or manual reset circuit breaker).
Try measuring the resistance of the trailer brakes when it's disconnected from the car - perhaps there is a short.
I find the wires on the magnets break off regularly (experience from high mileage hire Tvan's) - perhaps one has broken and is shorting out on the brake internals. [yes, I use proper Alko off-road magnets - I've got a collection of magnets with wires that have broken off at the edge of the magnet well before the magnet had worn down. I think Alko's magnet is a crappy design and the wire needs more support as it exits the magnet because I suspect the wire inside bounces around over corrugations causing it to break where it is fixed going into the magnet]