I'd wager a coupla bux on the dimmer switch. I've had several of them fail on me like that. Once I had one in a 67 Camaro just quit while driving down the street with the lights on. The cop behind me noticed it, too.
Luckily, I stomped on it a couple of times and got the brights working just as he was coming up to my door after he'd pulled me over. I explained what happened and he just suggested that I get another switch before I went out for any more night time drives. I went home and swapped the switch out the next day and never had any more troubles with that car. Matter of fact, I just sold it a few months ago....we had 2 67s and a buncha medical bills, so one of them had to go. Oh...the lights still worked fine when I sold it.
