Actually, my wiring is in pretty good shape. My Mustang went through a partial restoration before I bought it in 1999 and the under dash harness was replaced at that time. I can't take credit for the idea. A fellow member of my local Mustang club and a well known authority on 65-66 Mustangs (anyone else know Richard Porter and his sweet 66 K-Code?) has done this for several other folks who wanted to upgrade their early Mustangs to halogen headlights. One thing he did tell me was to definitely go with the more expensive 69 original Motorcraft switch. It's much more expensive than the current repros, but it's also much better quality. I got mine from Rockauto.com, but I'm not sure if they have any in stock right now.
1965 Convertible 200 I-6, C-4 Auto, Long Console, 6K Rally Pac, Power Top
1969 Mach 1 351 (W) 3-Speed Manual, Winter Blue, Deluxe Interior, All original.