Keep your tools in place - we all do something like this from time to time.
Mitch wrote: “A good place to 'grab' 12V at the back of the car is the B+ connector on the alternator, which is connected via heavy cables back to the battery (via the starter). Just remember to use an inline fuse with whatever you connect there, as there ain't none twixt there and the battery.” (My italics)
If you don’t use a fuse on anything connected to the B+ terminal, you have a potential there for 600 amps to ground, turning the wire you’ve connected into a lightbulb filament if it gets shorted to ground. It’ll go up so fast there won’t even be much smoke.