I always run directly to the battery and have a couple of fuses there. I trust 10g wire a lot more than 14-16g the auto manufacturer uses to carry the amps. It may be easy to tap into the auto wiring, but I don't think it's the best way to do it.
Most newer cars have an axillary fuse socket available on the fuse block. I find it easy to use this in lieu of a separate fuse holder. Make sure that the current available is high enough to operate your transceiver. A proper fuse is a proper fuse no mater what the shape size or holder.