Looks like a self resetting circuit breaker. if you clean it up you may see a 20 or 30 amp rating on it.
Not sure but some of the older trailers had an indicator light that could be seen in the mirror by the driver when the brakes were applied. If so the second wire on the blue terminal may be that light.
Considering the age I would replace it.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles