There could be several causes for this problem, and without being able to see and troubleshoot the brake light wiring circuit no one can tell you the specific cause.
Some of the possible causes include:
There are probably a few other causes that I have forgotten about.
The light switch.
The switch was stuck between high and low beams. Just flick the high/low beam switch once and the circut will be back to normal.
headlight switch
the reverse switch in the transmission
brake light switch bad check for continuity at the fuse then the switch, but yeah bad switch. Kongkit Raggan-Supatanampon
It would help us to know what make, model and year you have.
try the multifunction switch located on the steering column .
check the bulb again, are BOTH filaments good
Very possible ... best to check the wiring with a voltmeter checking for +12vdc.
check the ground on your tailights R.W: Have you checked the turn signal switch? That may have to be replaced.
Did you replace the fuse? have you checked the circuit board well the lights go on cause they tend to short out often
fuses are ok. replaced ignition multifunction switch and hazard, turn signal, cruise etc. switch. checked grounds. hazard lights when ignition is off but not in the on position. thanks Joe lapointe