re-wiring. 3 wires I question

is this a painless, or some sort of generic harness?
(im guessing it is) if so, the 3rd brake is for a brake light that is going to opperate independant of the turn signal (your mustang has only 2 wires going to the tail light, a running light, and a turn/stop light... the 3rd brake wire takes the turn signal out of the circuit...
dome light... most cars have the dome light wire ran through the b pillar... at the back (sort of) the car, yes, this is your courtesy light
no electric fuel pump on a stock mustang, so unless you plan on running one, either screw a wire nut onto the wire, and tuck it up into the quarter panel, or tie it up somewhere you can get to it in the future, you may want to use for, say amp remote or something... just leave the fuse out of the fuse box, so if it would happen to short, it wont cause you any grief.