I'm happy to report, it works perfectly! I wired one up with parts from my bin, using a reed switch, and it works exactly as expected. The biggest load I had here was a fan that draws 2.5A @14v and the MOSFET stayed nice and cool, I'll have to get it over to the test car across town to try it on the higher load. I also ordered a handful of hall effect sensors and I'll try swapping out the reed switch with the hall sensor to see how that goes too. Thanks again everyone!