What willcause the brake lights not to work in a

If it has bulb replace it.

Sounds like the switch is defective.

no. It will be either the switch at the brake pedal, fuse, bulbs or wirng somewherein the brake light system.

check brake light switch mounted on pedal arm.

The brake lights stay on even when the vehicle has been restored to the "before" condition?!? Now that's strange. I'm assuming the brake lights worked properly before you did anything with the trailer hookup. Is that correct? Do the brake lights work OK when a trailer is hooked up to the trailer lighting harness? Regardless, if the brake lights stay on all the time, there is a continuous 12V to the bulbs to ground. You have a wiring short somewhere in the vehicle. Have you checked the wiring for bare spots (caused by rubbing) or cuts in the wires.

Since the brake lights won't turn off, you have created a continuous path of 12V through the bulbs to ground, regardless of the brake switch operation. If you did not use the plug in type of wiring harness to attach the "trailer plug", then I would systematically undo each wire that you attached to and see what fixes the problem. I assume you know that the Blazer has a trailer connection plug-in location tucked up under the rear bumper. You simply unplug the mating harnesses, plug in the trailer wiring connector, plug in the disconnected harnesses and wha-la you should be good to go.

Hope helps.

the abs light will come on when you turn key on and go off after engine starts (3secs),it will also come on if you brake harder than required,to stop the wheels locking.If it comes on at other times it could be reservoir needs topping up or an abs fault(check it by standing on brake pedal at 30kph(wheels should not lock up until 3/5kph.)read antilock brakes or antiblocking brakes, 2 different systems.

Check wiring continuity from bulb to switch.Test if you have voltage reading at rear when brakes are applied.

Check the brake light switch, under dash, near top and on back side of brake pedal.Might be stuck or bad.

i would check it,seems u have an open circuit,switch not working,broken wire or corroded elect.connector.

There is a brake system balance valve that has a wire going to it. Typically near the mater cylinder where you put the fluid in. Remove this wire from the valve. ( You will also see 4 brake lines going in to this valve) If the light goes out, there was an imbalance in the front to rear brake system that tripped the light. You need to find the cause of this. This happens most often when someone replaces the front or rear brakes, and is pumping the brake pedal to get the pads or shoes seated onto the rotors or drums.

Getting the light to go out is a matter of bleeding the brakes, during this process you will cause an imbalance in the opposite way that caused the light problem to start with.

You need to find the cause of this light going on. Have the brakes inspected by a qualified garage telling them what happened

Good Luck!