brake lights ONLY work while headlights are on?

Parking lights should come on with the headlights. Brake lights work through the turn signal switch under the steering wheel. If you have a copy of the 68 workshop manual, look at the wiring there and follow it from the brake light switch under the dash to the back. Good luck.

68 GT500
68 1/2 CJ Coupe

MCA# 18519

I have a similar problem.you need to see if you are getting any juice between the backlights and the fuse box,if you are its prbably the turn signal switch,good luck

Try checking your grounds from the sockets to chassis too.

'66 A-code Fastback (therapy)
'89 Town Car (SWMBO D'driver)
'87 Caprice (as req'd by Hagerty)

Please be specific about brake lights: those are the lights that come on when you press the brake pedal. If that signal only works when the headlamps are on, you'll get doubled brightness. The two signals are entirely disconnected from each other, which is why you have a double-filament bulb. It's possible that the filaments are crossed/touching, and the brake light filament shorts out to the headlight side.

http://clubs.hemmings.com/baymustang/platesmall.jpgLet me check your shorts! My multimeter is just a-waiting! Formerly known as Midlife in the old VMF.
King of the Old Farts *struts*