Painless Performance Products introduces the No. 20120 wiring system for classic Mustangs, Falcons, and Comets. The kit consists of a main wiring loom (underdash), a firewall-to-headlamp harness (underhood), a taillight harness (from underdash to trunk area), and the instrument wiring loom. Most of this system has plugs that are compatible with your Mustang's switches, lamps, and accessories. The rest of it must be connected with crimped connectors and the like.