Truck cap lights.

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I recently purchased a used Jeracho truck cap for my 2011 GMC Sierra. The truck cap has a third brake light and a dome type light with a switch on it. The wiring for these lights runs into a 4 prong type connector similar to trailering connectors I have seen. This connector sits just inside the door of the truck cap on the left side.

My truck has a trailering package with the 7-prong connector. Does anyone know an "easy" way to connect to the 4-prong connector in the topper to get power to those lights? On my old truck I remember splicing into the parking light wiring to get power to a dome light. Just wondering if anyone has any suggestions.
 
Trailer connector won't work. The pins for the brake lights on a trailer connector are also for the turn signal lights, so you would have a third brake light that also flashes when you use a turn signal if you tried to use one of those. There's also nothing on a trailer connector for a dome light, although a 7 pin connector has a constant 12V pin. If there's a switch in the cap to manually turn the dome light on and off, that could work.

A lot of trucks have a connector hidden somewhere specifically for connecting to truck caps. I would search the internet and find out if your truck has one.
 
On my cap I popped a taillight and tapped the brake light using a Scotchlock. I made sure to go several inches back from the lamp assembly in case the tap failed; if it failed I was going to cut and solder. The two wires then come out through an access hole in the bed, near the cap door. The trailer lights are a ways away, and I'm not sure I'd want to cut any wire strands going to the trailer with a Scotchlock, as some trailer lights can pull some amps.

Didn't think to tap the parking lamp circuit to power a dome light. I figured when I got around to it I'd get a battery light. The battery would always be dead, but then the truck battery would never be.

On the 7 pin, isn't one of the wires backup? On your late model truck, if you open a door does all the lights, including reverse, come on? I'm wondering if you could just wire your topper into two connectors: one 7, one 4. Plug into both, with redundant connections. When you hook up a trailer, unplug the needed connection. That way the topper works as intended. Not as clean but then nothing got cut either. Edit: if you use the 7 pin then you'd lose the interior light in this setup, so that is a drawback; but if you use the 4 pin (pulling a utility trailer say) you would not.
 
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^^Tapping into a brake light wire only works if the truck uses separate bulbs for the turn signals. Otherwise, you have the same problem as using the trailer connector: the third brake light will flash with the turn signal. Unless of course you tapped into the third brake light on the truck, not the other brake lights.
 
Good point; forgot about that. I forget how I hooked it up now... I think I have ambers for turns, but it's been a couple of years. Pretty sure I didn't over look that detail (but now I want to look).

Could tap both left and red brake lights, and diode connect to drive the 3rd brake light. If it's an LED strip the lights probably won't care as much if incandescent.
 
Update: I purchased the 4 pin connector to hook up to the truck from Jeraco for $7.95. It has a red wire for the third brake light, a brown wire for the dome light and a black for ground.

I think the easiest way may be to tie into the 7 pin trailering wiring under the bumper near the spare tire. The 7 pin on my truck is labeled. Red is + auxiliary so I thought I could tie in the dome light to that. Blue is electric brake so thinking I can tie the brake light on the cap into that although there are two other pins labeled rt and lt stop turn. I am thinking I should not tie the brake light into those as the brake light on the cap might flash when I put the turn signals on? Does this sound correct?

My next question is what is the best way to tie into the wires? Supton mentioned a Scotchlock. I am concerned about the winters here in PA and all the slop that splashes up in the area I want to tie the wires into. Do people use a lot of electrical tape or possibly heat shrink? Any comments or suggestions welcome. Thanks
 
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