I would try look at your brake light switch under the dash. Most of the time you can adjust the switch, There is an arm that you can adjust or could be a switch that you can adjust. Follow the brake peddle you should see an arm or a switch that can be adjusted. While adjusting it if you go the wrong way the brake lights will stay on all the time. you should adjust so that you have to press the pedal down at least 1 inch before the brake lights come on. Good luck Craig

Here is a useful website that has a schematic of mustang wiring. [url]http://www.hammar.dyndns.org/wiring/wiring1967.htm[/url] You also might want to check out the Turn Signal Switch because just about everything to do with external lights runs through this.