This really got me stumped. My 97 Blazer keeps blowing the break light fuse. I have been chasing this demon for a week. Have inspected the wiring harness from front to back no pinches or scuffs I've even dropped the fuel tank no damage. The thing that gets me is if I disconnect the fuel pump harness at the firewall the break lights work fine no fuse problem when I plug it back in fuse blows. According to wiring diagrams they are 2 seporate circuits one break/chime the other ECM/Bat it doesn't make sence I can't find a pinch or scuff, will continue looking need suggestions and help please. It even does this with both tail lights unplugged from the harness the high breaklight is unplugged also.

You say with the harness unplugged at the firewall unplugged the fuse doesn't blow but the brake lights work. I would take a test light and find which wire on that plug gets power to it when the brake pedal is being pushed by doing that and figuring which side of the plug the backfeed is coming from you can fix you short. If the brake light wire rubbed against the wiring to the fuel pump at the rear of the truck and is backfeeding there then power will be on the chassis side of the plug when the brakes are applied. By doing that you'll narrow down the area to trace to one wire going one direction. If you will post a schematic or e-mail it to me I can walk through it with you and get it fixed.

Thank you I never thought about a back feed. I have a digital multimeter that does dc and will follow your wisdom. will try to get the schematics scanned and posted tomorrow evening as I have to change the water pump (weep hole is leaking) on my wifes Nissan tomorrow She who must be obeyed has ordered it complete tune up too. I feel like a smacked azz being a master electrician with ac voltage. DC voltage in cars get me.

have u put a fuel pump in it lately, if so the harness is probably smashed between tank and frame rail

Found the short it was the little white wire in the factory trailer wiring harness. I guess it's for electric trailer brakes and was grounding out as who ever had it didn't properly insulate it after it was cut. Thank you okbassforum you put me on the right track to finding it as the inside part of the plug sent me to the back looking for the little white wire and it is only in the trailer harness. Again THANK YOU it was driving me crazy.

I have repaired all sorts of wiring problems, including stuff other techs couldn't figure out and a truck that got stuck by lightning recently.

Glad you fixed the issue????. But a white wire would be a ground. I would continue looking to find where another wire is shorted to the white wire.

ChuChu for some reason on this 97 2 door 4X4 the factory trailer harness is seporate from the tail,break,and turn light harness. I do agree whites are usually ground as the ground is a bigger white wire which is why it drove me nuts. Maybe the harness maker ran out of the proper color. The factory trailer harness is part of the fuel pump harness and not the tail lights they are totally seporate. One more thing the factory used black for fuel pump ground. Maybe GM being jackwagons is all.

Every maker uses different color codes some use black for ground some use white. You just have to read the schematic each and everytime to know for sure. I've worked on vehicle where all the wires where 1 color and you had to read the numbers printed onto the side of the wire to know which circuit you had.

Sorry for ruffling feathers, but I would find out what that white was is for, and why it is hot. Maybe you solved the problem, maybe only postponed it. Maybe that's my problem, but I would feel better to be safe than find my vehicle burned to the ground.
BTW, my 87 Blazer trailer wiring is in the same bundle as the fuel pump, and the white wire is ground.

OK, on your model, GM uses a white wire between the brake light switch and the turn signal switch. So naturally the trailer brakes would come from this wire. The trailer harness is tapped into this white wire. If you would have said it blew the fuse when you stepped on the brake, it would have helped.

No feathers ruffed up here I would not want to be sitting at a light and it catch fire BBQ Brat not good. Have a great day and again thanks for the input and help.