What would be the best way to wire the power for a truck cap interior dome light? I spliced the ground wire into the truck cap brake light ground wiring, and I'd like to power the dome light so that it only can be used when the key is in the 'on' position. There is also a switch at the dome light itself so I can turn it on and off from the light. I have the power wire routed from the dome light to behind left tail light, but I don't know where to route and connect it from there. What would be the best way to route the power wire so that the dome light can only be used when the key is in the 'on' position? I also bought one of these from Amazon, but I don't fully understand how to use it.

 

To use a tap like that you unplug a Key ON power fuse, you then need to test the fuse box terminals for that fuse to see which terminal has 12volts with key on

You need to plug in the Tap so the WIRE is opposite the 12v terminal

Install the original fuse on the lower slot, and new fuse(for light) in the upper slot
Run wire from fuse box to the Plug on the bed side, you do have a disconnect plug so you can take the cap off without cutting wires right?
You need a disconnect, trailer wiring plugs work fine for this, 4 wire, so power, brake, ground

 

Yes I do have a 4-wire connector, but I'm only using three of the wires. One wire for the cap brake light power (green), one wire for the dome light power (brown), and a shared wire for the cap brake and dome light ground (yellow). The fourth wire (white) isn't being used for anything. The cap brake light + wire (green) and the shared ground - wire (yellow) are spliced into the left tail light brake wiring.

I'm also having a problem with the wiring. I connected the dome power wire (brown) to the fuse tap wire, and replaced the radio fuse (15a) with the fuse tap so that it only works with the key turned on. However the dome light doesn't turn on with this setup. I know that the dome light works because it turns on when I connect the dome power wire (brown) and the shared ground wire (yellow) directly to a 9v battery, however it doesn't turn on when I run the dome + wire (brown) to the fuse box and have the ground wire spliced into the left brake light ground wire. The cap brake light also works fine with this setup. Does the shared ground wire only work when I step on the brake pedal? If not, then I don't understand why the dome light doesn't turn on when I connect the dome + wire (brown) to the radio fuse and turn on the key, since the radio also turns on. I know very little about electrical wiring so something must be wrong with my setup. Please let me know if you need more info.

 

I would get a test light, or volt meter to test you if you have 12v at the tap, maybe its a bad part, does the radio still work?

And do you have both fuses in the tap?

No, grounds can be shared, so that wouldn't be a problem

 

I figured it out. I had put the 15a radio fuse in the fuse tap and then put the fuse tap where the radio fuse was located (#29), but the fuse tap needed both the 15a radio fuse plus another 5a fuse to turn on the dome light. It works now, thanks.

 

Yes, the 5amp fuse powers the added circuit, dome light

Good work

 

I'm still having problems with the electrical wiring, the 5 amp fuse in the fuse tap keeps blowing every time the dome light switch is turned off. The dome light bulb turns on if I reconnect the car battery with the light switch in the 'on' position and the fuse tap connected, but the fuse then blows/snaps as soon as I turn off the dome light switch (and it won't turn back on). I'm fairly certain that the problem is the wiring setup of the dome light switch, because I have tried different fuse taps, different wires, and different fuses with the same results. I attached a picture of the wiring setup inside the dome light switch.

Cap Brake and Dome Light Wiring

Fuse Tap

 

You have a 15amp fuse on the dome light wire(red) on fuse holder
The yellow fuse location should be same amp rating as original fuse in that fuse panel slot
So reverse the locations in the holder

 

The same thing happens when I reverse the fuses (15 amp in front near the blades, 5 amp in rear). The rear fuse always blows when I reconnect the car battery. I already replaced the wiring around the dome light once because they melted, so something in my wiring is wrong, or could the switch be bad? I have also tried different fuse taps with the same results, so the fuse tap isn't faulty.

Is it my wiring? The dome power wire to the switch is combined with the dome power wire to the bulb, which then is combined with the dome power wire going to the fuse box. The dome ground from the switch is combined with the dome ground to the bulb, which is then combined with the brake ground wire, and that is all spliced together into the tail brake light ground wire. I have tried connecting the wires to a 9v battery and the bulb turns on and off properly, however when the bulb is off the wires get hot very quickly, and I don't know if that's normal.

 

No, you have a short in the new dome lights switch, its defective, its connecting 12v to ground directly when in the OFF position