Cap interior won't light

This has got me baffled! I have power to the light bulb contacts in the light fixture(12 v). The power switches off and on with the switch. The light won't light when bub is installed. Tried two different light fixtures. Neither one will light when installed. Both lights light when touched to the battery. I even flipped the wires and both lights light when wires are connected in either polarity to the battery! Any suggestions????

 

You say you have power......are you testing for both power and ground at the actual light contacts? On a second gen truck about the only place you are going to get a good ground in the bed area is at the metal rear corners around the tailgate, everything else is plastic.ou

Edit: Read your vehicle info and see that you have an 04. I would still ask about the ground, on my old 1986 Yota pickup, the bed was very poorly grounded, a camper light would work, but very dimly when grounded to the bed. I eventually grounded it to the frame.

 

You have to add a resister or buy mine with resister already attached. To test put led in and turn key to acc. Dome will light. If led doesnt light then flip 180 and try it again.

 

I actually inserted the two leads of the VOM into the light socket. I've talked to two different guys last night and one suggested locating a new ground wire and the other said he had a similar problem with his cap light and that the installer had to run new wires from the battery as there was a slight voltage drop that somehow faulted the connection. I did have a slight difference 12.4 v at the battery and 12.2v in the back. So I guess I'll try a new location for the ground wire first then simply run new wires. Thanks for the response though!

 

The cab light. This is what you are talking about, correct?

 

He is talking about his light in his camper shell.

 

Ahhhhhhh.