it is always corrosion I would clean the terminals/contacts where the bulb meets the source. If there is some kind of spring involved make sure it still works. Sometimes the spring has lost the ability to create presure on the terminals.

Try something like steelwool, emery cloth or sandpaper on the terminls where the bulb contacts the fixture.

Thanks guys. I think I will let Sky Guy come by and try to help me figure it out. I am sure the issue in the rear light is corrosion, as I do have 12 V at the contacts inside the fixture. The front may be yet another issue.

I KNOW I will regret saying this, but at least my tralier lights work well.

Now yer FUC%ED!

Yup, cursed you are.

Dave, Did you check at the bulb or the contacts of the fixture? A reading at the bulb would indicate an open in the circuit (bad contact or bad at the bulb.)

I have to fix a bad running light on my trailer. The wire looks to be fried. The copper is black between the wire and the insulation. Might have to rewire it. The wire was loose were it corroded away from the connector.

Dave, 12 volts is available. Thats not the same as CURRENT.

Once the current passes thru the light bulb, the 12 volts is gone. If the light is bad or has a bad connection, you can run a test light or volt meter with the red on the hot side of the bulb and black touching the cold side wire. Dont touch ground directly with the black or it will register positive.
The hot lead into the volt meter or test light will register 12 volts if you touch ground.

Do touch those two areas and it will tell you if the ground is good.
You can also switch to OHMS and measure the resistance ONLY with no power going to the circuit. Touch the wire down stream of the bulb and touch the end of the ground wire or touch the frame. You should get a beep. (test it before and touch the two leads together while set on OHMS) You should get almost identicle result. If not you have resistance or the open in the wire.

corrosion causes resistance. In the electrical equation, resistance is going to reduce voltage going to the light and it will not light up. Two bulbs can accomplish the same thing, but in most cases the two bulbs will be dim and yet the voltage will be used up, by the time it gets to the ground.