There is no fuse for the dimmer circuit. It's connect directly to the body control module. It actually works a little different. It uses a constant +12v power source with a varying negative (ground) which allows for the dimming. So when it's off both sides of the bulb actually have +12V (potential difference of 0V). So if you want to tap into it you need to run a constant +12v source and use the dome light wire as a ground
With regard to the original question, is it best to use an amp's high level (speaker) input or a quality converter to send the signal in via RCA first? Or not much difference for a sub?

