My brother wired lights under each side of the dash, and under each seat, total of 4 lights. He used a lead from the dome light to power on a relay, in turn powering the lights. (I think the lights are negative switched, making wiring a little confusing). He also had a switch on the dash to power them on at will, otherwise they would only come on when you open the door. So, they aren't dimmable, but they were not obnoxiously bright, just a nice ambient glow.
He ended up replacing them with LED's when he put LED's in the dash.
Hardly any of the factory wires are big enough for the load they come with stock, much less anything added to them. I'd say use relays. He even rewired his headlamps with relays- particularly so he can run the high and low beams simultaneously. I plan to do this with my truck as well, its amazing the difference it makes for close in visibility while traveling a dark road. The high beams tend to miss the overgrown rodents standing on the edge of the road. Here I am off on a tangent :-laf ... hope some of that helped.