Why won't my add-on LED lights work? *SOLVED*

[M37owner]

I bought a small (8"L x 1/2"W) 12V LED light array to mount under the rear bumper (sort of a low mount 3rd brake light). The LED is supposed to work from 6V to 18V - and they work fine connected directly to the battery, but when I tie it into the cars' RR brake light wiring, it won't come on. I have 11.2V at the connection at the rear LED brake light connector.
Perhaps I can't get enough amperage through the LED brake light circuit to power this add-on LED light strip. I have not worked on car with LED brake lights before but I assumed I would be able to tap off the existing brake circuit. Where did I go wrong?

[About Tree Fiddy]

Do you have the polarity correct? Unlike a standard incandescent light which can be installed without regard to polarity, an LED must be installed with the polarity matching since it is a diode or it will not function when reversed.

[M37owner]

Thanks for replying. The polarity is correct (I actually tried it both ways. And I marked the pos and neg wires when I tested the LED light on the battery). Since the voltage is sufficient, I must not be getting enough current flow. The power for the stock LED brake lights on the M37 comes from the body-control module, so I suspect that is limiting current somehow.

[anotheraznguy]

i have a lower LED on my rear bumper as well and it works fine. double check the polarity and test the strip again.

[M37owner]

i have a lower LED on my rear bumper as well and it works fine. double check the polarity and test the strip again.

That's encouraging. Did you connect it directly to the existing brake light wiring at the right taillight housing - or ?

[TDot]

Your avatar info says you two have different cars, might be different setups. I don't know, just want to make sure you too are aware of this. Unless anotheraznguy has an m37 that he didn't list. Anyway, good luck.

Edit: again i dont know what is happening, but as far as current, led really don't require much current...from the ones I've seen at least. Just look at the gauge of wire that should be feeding, and that will tell you if enough current is able to get to your led which shouldn't be more than 1 amp, two at most from LEDs I've seen. Im assuming the wire feeding is at least 16 gauge. Voltage is provided, current is drawn.

[M37owner]

Problem solved -- it was installation error. I did not have a good ground.
My ground was using a crimp-on eyelet under an existing stud. I was clamping onto the stud to check voltage - but there was a bad connection between the eyelet and the stud. So I ditched the stud-ground connection and sliced directly to the ground wire. All is good now.

Wasted a bunch of my time - and some of yours. lesson learned.

[anotheraznguy]

Congrats. Glad you were able to get it all sorted, now take some pics . The next step, if you choose to, is to make it dual filament style using resistors and diodes. Allows you to have it as both a brake light and parking light.