I did a search and didn't see this particular problem mentioned, so here I am. Yesterday, I noticed the interior light not working by any noticeable degree. The light switch has the three options, ON, OFF and DOOR. Moving the switch to ON, I couldn't tell if the light was on at all. Admittedly, I didn't try setting it to DOOR and opening the door before I bought a new bulb. I took the old bulb out and immediately noticed that one of the metal caps for the bulb came off in my hand, so replacement is not entirely unreasonable. I took the bulb with me and found one that was about the same size, although made of a pair of LEDs. Yes, I have read that LEDs tend to do wonky things in these trucks due to lower resistance. I do have the radio taken out of the truck right now, preparing to take the dash apart, clean vents, replace a cable and reassemble it all. I found that when I turned the switch to the ON position, the light is extremely dim, not enough to light up anything. When I set it to DOOR and open the drivers door, it works and is extremely bright. This tells me that fuses should be good, that it is at least working in both positions, but I can't explain the difference in brightness. To be honest, the light I get from the DOOR setting is too much, but the setting from ON is not nearly enough. Is there more than one fuse that controls interior lights? Is there maybe a short somewhere?
Door switches on most vehicles provide the ground connection for interior lights. So it sounds like your door switch is providing a good ground but the manual switch on the light isn't. Check how it grounds at the switch.
I took the bulb into Wal-Mart to compare it against what they had on the shelves. I bought the smallest bulb they had, but it's still a little too long for the mounts. I may try taking a couple pair of needle-nose pliers to put a dog-leg bend in the brackets so that it will fit properly. This morning, I checked the contacts with a multi-tester. I found better than 14v with the switch set to door, but a bit over 11v when set to on. I took the right-hand screw out and put it back in. Light now is plenty bright on either setting. I do belive that I need to clean the contact with a wire brush. I don't recall the part number. Cost me a little more than $6.00.
Original bulb would have been a Wagner 12100(CARQUEST is same #), GE DE3175(Sylvania same #). Pretty standard dome 10 watt light bulb on most Japan imports.
