Originally Posted by MedSailor
I know that you're supposed to choose your fuses to the size of your wire, not the device amperage draw.
Not necessarily. It can be either way depending on the situation. Because of the wire sizing requirements of 12 volt circuits, the wire can typically handle way more current than the load requires.
How much current a wire can safely handle depends on the physical size of the wire, the temperature rating of its insulation, whether its run with other wires and how many, and whether its in an engine space.
In your case, if this circuit is soley for the heater, I would size the fuse for the load of the heater which is far less than #8 or #6 wire can safely handle.