My guess is that it would be o.k. to tap up front for 10 watts. Disclaimer: I am frequently wrong. My theory: 10 watts / 12 volts = .833 amps, which I would think is a small enough added load.
Regarding tapping at the tow pigtail: (1) mine has a wiring diagram on the hinged plastic cover (2) once you locate the correct wire, it could also probably be tapped up front somewhere as I assume the circuit has fuse/relay in the underhood boxes.
I live in Larkspur.