Tail light won't work but brake light will

Justo change the socket or put some grease for bulbs

Check the socket for power (with a 12 volt test light probe) to the pin that operates the running light bulb filiment, use the other tailight for a guide or just touch all the pins, one should light the test light up

1. Replace your car tail light. You will need working gloves to protect your hand during the process. Of course you will also need a screwdriver, cleaning cloth, wire brush, and a good set of tail light bulbs.

2.Understand that different models of car have different set of tail light covers. Naturally, these covers will require different un-installation procedures. So, determine if you can dismount the cover from the outside or you will have to dismount it from the trunk of the car.

3.Determine how to dismount the tail light cover, carefully detach it from the car. You can use different sets of screwdrivers for this. Be careful not to break the tail lights cover. The important thing is you must be able to expose the bulb in order to extract it.

4.Remove the tail lights cover, unscrew the bulb from its socket. Sometimes you will have to press a little and twist the bulb in order to extract it from the socket.

5.Extract the bulb, bring it to your favorite auto parts and accessories store and find the exact match of your tail light bulb. Be sure to get the correct match because if you use other types of bulb, your tail lights may not work properly.

6.Reinstall the tail light bulb, clean the aluminum casing of the tail light. Wipe the socket and wiring with a dry cloth. You can use wire brush if there are corrosion around the tail light casing.

7.Put the good bulb on the socket. You might still need to apply a little pressure on the socket before you can install it fully.

8.Reinstall the tail light bulb, replace the cover of the tail lights and secure it tightly.

9.Test to make sure your new tail lights are working. Start the engine and apply the brakes. You can ask someone to see if the tail lights are really working.

resource: http://www.wikihow.com/Fix-Car-Tail-Lights

If you are saying a tail light does not work, and you have changed the bulb, have you checked for voltage and ground at the socket ?
Could be a faulty socket or wiring problem.

there must be a short circuit which powers tail lights and brake lights. probably you have a dual filament bulbs which have both brake and tail light together. check for bulb, socket/or bulb housing and harness of that light.

THAT RIGHT IF BRAKE SWITCH BAD NO BRAKE LIGHT WILL WORK.SPRAY ELECTRICAL CLEANER IN BULB SOCKET BULB SOCKET COULD HAVE OXIDATION AND CORROSSION CAUSING POOR CONTACT.

first eliminate the possibility that you have the wrong bulb in the wrong socket. it sounds dumb but going way back to 1157 bulbs there have been two rear tail light bulbs which will fit into the same socket but are electrically different. If you have ever driven down the road following a car which had one bright and one dim tail light, or brake light, then you have seen this in action.

since you have one good tail light assembly remove the bulbs, taking great care not to mix them up and make sure the bulbs are exactly same as in the non working assembly. i mean you have to look closely at the bulb's connection points as they will look very much the same.

if the bulbs are correct then you are looking at buying a new socket, or light assembly if the whole thing is wired into the car.

You want to change the tail light bulb?

The process for replacing tail light bulbs is the same in all models. If you have a burnt-out tail light bulb, you may need to replace the brake light, the reverse light or the turn signal. The procedure to replace each of the bulbs is the same.

Instructions

1. Open the Hummer's rear swing gate. This will expose the edge of the vehicle's tail light assembly. You'll see two small Phillips head screws that hold the tail light assembly in place. Put on your gloves.


2. Remove the two screws from the tail light assembly. Pull the tail light out and inspect the back for two bulbs. The top bulb is the brake light and turn signal. The bottom bulb is the reverse bulb. Determine which bulb you need to replace.

3Rotate the bulb's socket counterclockwise to disconnect it. Unplug the bulb from its socket.

4Push the replacement bulb into its socket firmly. Insert the socket back into the tail light. Turn it clockwise to secure it in place.

5Place the tail light into its cradle. Hold it with one hand and replace the Phillips head screws with the other.

CHANGE OUT THE SOCKETS THE BULBS GO IN AND CHECK THE FUSES ALSO GOOD LUCK!

Check the relays associated with the tail lights, I believe that the tail light relay and the turn signal relay are two different entities. Hopefully that helps.