L.e.d. Third Brake Light Wiring

[gilroy222]

Hi. I have an 00' Sierra single cab and i just got a set of projector headlights, bumper lights, tailights and third brake light. everything wen it fine excpet for the third brake light. I pulled out the stock one, undid the harness and got to looking. My new l.e.d 3rd light has 4 wires coming from it(2 power and 2 ground). My problem is that I have to take out the stock cargo lights and put them in the new 3rd light assembly but I can't figure out where to splice. The stock harness has 4 wires to it. The brake light wires are orange and the grounds are grey. I used a test light on the stock harness(coming from the truck) and one of the pins had constant power, the one diagonal to it had power when i stepped on the brake, and the other two pins did nothing, even when turning on the cargo light. So i have no idea where to wire these things into because the brake light never came with instructions or anything. So just wondering if anybody has installed a 3rd brake light before? and how did you do it?

[minibike132]

Sounds to me like the brake is power switched and the cargo light is ground switched. Meaning you should have constant ground to the brake circuit and an open circuit on the other ground untill you turn on the cargo light.

[gilroy222]

Sounds to me like the brake is power switched and the cargo light is ground switched. Meaning you should have constant ground to the brake circuit and an open circuit on the other ground untill you turn on the cargo light.

 

 

So how would I wire that up? I just don't have very much knowledge on this kinda stuff. Would I cut off the cargo lights from the stock assembly then wire them into the two wires on the harness that don't do anything? because remember even when I turned on the cargo light button, the message center displayed "cargo light on" yet the wires weren't making the light turn on on my power light.

[cgalford]

Expanding on what sparkstech said, this is what it sounds like you have but your going to need to test for ground on the other two wires to be sure:

 

Constant 12V goes to the cargo lights +

 

12V with brakes goes to brake light +

 

(Need to test the next two to make sure this is correct!)

 

ALWAYS ground goes to brake -

 

The other "ground" should switch between ground and open and this goes to cargo light -

[gilroy222]

Expanding on what sparkstech said, this is what it sounds like you have but your going to need to test for ground on the other two wires to be sure:

 

Constant 12V goes to the cargo lights +

 

12V with brakes goes to brake light +

 

(Need to test the next two to make sure this is correct!)

 

ALWAYS ground goes to brake -

 

The other "ground" should switch between ground and open and this goes to cargo light -

 

 

ok that makes perfect sense. but how do i test the ground to see if it switches?

[cgalford]

Find a known +12 source for your positive lead and with the negative see if one of them gives you a 12V reading.

Expanding on what sparkstech said, this is what it sounds like you have but your going to need to test for ground on the other two wires to be sure:

 

Constant 12V goes to the cargo lights +

 

12V with brakes goes to brake light +

 

(Need to test the next two to make sure this is correct!)

 

ALWAYS ground goes to brake -

 

The other "ground" should switch between ground and open and this goes to cargo light -

 

 

ok that makes perfect sense. but how do i test the ground to see if it switches?

 

[gilroy222]

Find a known +12 source for your positive lead and with the negative see if one of them gives you a 12V reading.

Expanding on what sparkstech said, this is what it sounds like you have but your going to need to test for ground on the other two wires to be sure:

 

Constant 12V goes to the cargo lights +

 

12V with brakes goes to brake light +

 

(Need to test the next two to make sure this is correct!)

 

ALWAYS ground goes to brake -

 

The other "ground" should switch between ground and open and this goes to cargo light -

 

 

ok that makes perfect sense. but how do i test the ground to see if it switches?

 

 

 

 

ok so could I just touch the positive part of the test light to the positive battery terminal, and touch the negative to one of the grounds on the brake light and hit the cargo light button?

[cgalford]

procedure would look like this: with positive lead on battery + touch ground to both negatives. One should show 12V on the meter the other nothing (or a bit of noise but not anything near 12V) then turn on the cargo lights and you should be able to read 12V on both. The one that changes is the ground for the cargo light.

[gilroy222]

Ok guys I have everything wired up and installed. The brake lights work beautifully. The only thing is when I was doing the ground test something happened that made the ALWAYS positive stop providing current. Meaning when i switched the test light around to test for positive, nothing came up on the wire that should have been providing constant power. So I hooked everything up anyway and sure as I thought the cargo lights don't turn on because of this. I assume it's a fuse, does anyone disagree? Anyways I appreciate all of the help you guys offered. I will be getting a new fuse for the cargo lights and hopefully that fixes that. If not oh well.... the brake light is the important thing.

[gilroy222]

Ok so I just realized now that my dome lights do not work??? whenever i try to turn on my dome lights the message center displays "cargo lamp on", the dimmer switch also activates the message however the cargo lamps do not work as there is no power running to them. Does anyone know the code of the fuse so i can try and replacing them? Are the dome lights and cargo lights wired together? maybe when I was wiring up my 3rd brake light i f**ked up the wire..... hopefully a fuse will replace this??