With my mods, I'm also going to put my heater in the rear seat thigh support. Not exactly sure how, but thats my plan!
As for electrons.
Just because theres a 10 amp breaker, doesn't mean the device is drawing 9.999 amps. A 7.5 amp breaker would probably be OK, however, I think a 5 amp breaker would be prone to false tripping (the blower draws about 5 amps, depending on speed and load).
The formula for Current (I) is;
I = E/R
I=Current, E=Voltage, R=Resistance
Keep in mind that this is a static formula and doesn't take into motor load and motor speed.
Remember, The breaker isn't protecting the device, its protecting the wiring. The wiring is capable of safely supporting 10 amps (probably more like 20amps, depending on distance).
I also figure in what breakers I have on hand. In the above example, I most likely had a hand full of 10 amp breakers. IF I would have had a hand full of 7.5 amps breakers, I probably would have used one of them.
When I select a breaker size, I first determine the maximum load that will be placed on the circuit. If its small i.e less than 10 amps, I generally double the wire size required and the breaker size. As the load increases, I'll generally do a factor of 1/2, again, make sure the breaker pops before the wiring deteriorates.
Waiter