I agree with Tone and the Bear, DO NOT tap wires. I did like Tone stated and ran wires to the fuse block (Driver's side, side dash) and used this:

"Add -a- circuit" By Littlefuse

(click the pic)

This allows you to tap into an existing circuit and keep the original amperage fuse for the circuit and an aditional small amp fuse for the accessory. You can find them at Checker, AutoZone, etc.