How can I restrict current to 12 amperes on a 12V circuit?
A PWM motor controller is the best solution. It needs to be mounted where is is not subject to splashing or designed with a sufficiently sealed enclosure. Mounting electrically insulated power devices to a heatsink that has fins protruding from a sealed enclosure is one approach. The size of the enclosure must be large enough to safely dissipate heat from the lower-power components.