Floorboard
I would guess the headlight dimmer switch on the floor board is bad or has a loose wire. Possibly the head light switch is bad. The headlight switch has a separate circuit breaker for just the head lights.
in glove box
The cheapest place to buy a 1966 Ford Mustang might be through a private dealer. It may be possible to buy a Mustang for cheap that needs some work done on it, which will lower the price considerably.
Unscrew bezel and switch should slide out through the rear (under dash)
Bad ground or bad switch
ford mustang 1966
The top speed for the 1966 Ford Mustang is around 150 to 170
http://auto.howstuffworks.com/1965-1966-ford-mustang-specifications.htm
How fast does the 1966 Ford Mustang Shelby GT 350 do what?
Those switches are all located where the shifter attaches to the top of the transmission, on the shifter iself.
Research has shown that there are a number of options available for purchasing parts for a 1966 Ford Mustang. These parts can be found on eBay, CJ PonyParts, and Dallas Mustang.
The back-up switch is located on the transamission near the back, it is activated by the shifter rod when going into reverse. :: AverageJoeResto.com ::