That happend to ours, they had to replace the board in the dash .. fixed the problem.
Check the brake light switch.
Check the fuses then check the brake light switch.
it has a bad turnsignal switch
i think that the light switch is shorting out are the auto light switch shorting for the thumb switch
It is located under the drive side dash. The brake pedal mechanism presses against the switch when you press on.
you can find the switch on the back of the brake pedal.
switch is on side of transmission under the car
If the brake light fuse is good and the brake light bulbs are good it's probably a faulty brake light switch.
You probably have a faulty brake light switch
Most likely the dimmer switch. I have had this problem before and I went to a pull a part junkyard and bought a few of them. I highly recommend doing this, as I have already blown 2/3. It's a common problem with our trucks.
try looking on the www.blazerforum.com
on the transmission