I have a 2001 2dr blazer... When i step on the brakes only the 3rd brake light goes on but when my foot is off the brakes the lights work fine, I replaced the bulbs and still nothing...

 

The lower brake light signal goes through the multifunction switch. Do your turn signals and hazards work? Does messing with the hazard switch affect the operation of the brake lights?

 

I have the same problem i replaced all the bulbs, fuses and brake switch. The hazzards work just fine. I still have that issue. The turn signals work as well

 

I seem to have the same issue.. I dont seem to have power to either of the brake lights.. but the turn signals work fine and the when the headlights are on the low brake lights are on..when the brake is applied nothing happens.. anyone know where there may be a splice or does this wire come straight from the turn signal switch??

 

I'm casting a second vote for the multifunction switch for both of you. If you're still on your first ones, consider yourselves in a minority that they lasted this long. My 2000 is on its third one. Both times the symptoms were different and also different from both of yours. I've been thinking it would be an interesting thread to have everyone list the symptoms their trucks had when the MFS went bad. Sometimes the trucks act downright haunted!

 

I had the tail light ripped apart using my meter and my helper( 5 yr old daughter) stepping on the brake to see if I had power back there.. alas nothing.. so going through all of the fuses and ripping apart part of the back to start chasing out wiring.. I decided to try the hazards to make things a bit easier... and voila.. they worked again.. power and brake lights worked.. I shut the hazards off .. it's like nothing ever happened.. the brake lights function like they should.. with the headlights on or off. so next step.. replace the multifunction switch.. funny back story though.. another symptom as well.. it took 3-4 days after we got the truck to figure it out. but there was always an intermittent ticking of the flasher coming from the passenger side same pattern as the turn signal but the lights never flashed.. everythign other than the ticking worked as it should.. one day I finally tracked it down to being the MFS the contacts must be worn inside... but now with this brake light issue.. I think its finally time to retire the original switch.. hey after 180,000mi I guees it could use some new parts..lol

 

You may be able to replace just the hazard switch for much less $$. Search on it and you should find the PN.