Fuses are designed to protect the wires, not the end item that's being operated. Things like heater motors will draw more current than other accessories, so the correct size fuse will carry the full load of the item being operated, plus about a 20% overload. The wire should be sized appropriately. For instance: Tail light circuits generally have #18, or maybe #16 wires, and they're protected with either 7 1/2 or 10 amp fuses, depending on how many bulbs are burning. A heater motor will probably have a #12 wire, and carries about a 20 amp fuse. If a #10 wire is fused, it's probably going to have a 30 amp or bigger fuse, because the wire is capable of carrying that much current without overheating. I'm not sure what size fuses were original equipment, but use a little common sense and be sure the wires are protected from over-current situations while still being able to carry enough current to operate the accessories they supply.
Jerry
