EE help! Powering modern LED third brake light for 911 retrofit

[422flat6]

My 72 911 doesn't have a third brake light and I want to install one. I know about the Daniel Stern Hella kits and I'm sure I could put in an incandescent 911SC unit, but I wanted to be different. 2006-2010 VW Jetta and Passat have a really great unit. Extremely bright, small, fits on the back glass of a 911 perfectly. It's got two wide plastic pads at rear glass angle that some 3M black foam tape will stick on perfectly to adhere it to the glass. (Hoping I didn't just drive the price of junkyard Jetta third brake lights up 200%)

I snagged one from my local junkyard with the VW connector and pigtail. It's got a brown wire and a black wire with red stripe. I assume brown is ground. I assumed I could just put 12V from my battery to it and it would light up. Wrong, apparently. I guess I need an LED driver. The back of the housing says 13.5V 4W. The LED board has 6 surface mount components with "3R3" printed on them and one larger component that has "GN1M" on it. Google tells me these are 3.3ohm resistors and a rectifier.

I tried putting 12V on it both ways with the car off... no light. I started the car and put 14.4V on it both ways and still no light. Any ideas? What are the chances I smoked it with my unscientific trials? Do I need EXACTLY 13.5V 4W (300mA) to make it light up at all? Is it bad? I can't imagine the failure rate of modern LED third brake lights is very high. The car I snagged it from was not rear-ended.

Surely there's an EE among us who can offer a suggestion on how to make this thing light up. It would look nice in my 911 and hopefully improve my chances of not getting rear ended.

Edit: Advice from non-EEs is welcome too of course!

[RSBob]

Would like to hear the answer to this

Call a VW dealer. Service dept and ask what it needs

[Porboynz]

Sounds like the rectifier/diode is there to protect the strip from voltage reversals and the 3.3ohm resistors limit the current. I count 18 LEDs so 1 x 3.3 ohm resistor per 3 LEDS. (3x6=18)
Based on approx 2V drop per red LED, I'm guessing there are 3 strings of 6 LEDs, you can probably trace the series parallel wiring on the circuit board the LEDs and SMD's are mounted on.

Anyway, you should get some light connected to 12V so either you have a poor connection somewhere or your LEDs are fried. Try a simple 1.5V torch battery across each LED if you can get at each connection.

[Quicksilver]

An LED driver is used to control the current across the LEDs instead of using the less efficient method of limiting current with resistors. You have resistors so it wouldn't use a driver.

I suspect it is toast but you should check to see if you are getting voltage to the strip. Resistor controlled LEDs can deal with some level of voltage variation so the difference between 12V and 14-1/2V won't be an issue. I don't see a rectifier in the pic but if it is in there to protect the circuit you might try just supplying the 12V to the strip itself to see if the rectifier is what failed. Just make sure you have the polarity right.

[422flat6]

Thanks. I got the multimeter out and tested individual components. Found high resistance at one of the resistors and re-soldered it. Works perfectly now on regular 12-14V power from the car. No need for a special driver or voltage regulator. Patched it in from the brake light switch at the pedal cluster because I'm OCD about not cutting factory wiring.

The angle of the mounting pads is an excellent match for the 911 rear glass. I have it mounted at the top of the window now, but there is a slight interference with the rubber window seal so you have to leave about a 1/2" gap between the top of the light and the top of the window. I may move it to the bottom of the window because I think it will look slightly better there since there won't be any gap at all. It's not bad up top though.

I especially like this unit because the lenses are good. It produces a nice solid bar of light instead of the discrete LED dots you see on many LED third brake lights.

I'll post some photos of my install as soon as I get it tweaked to my satisfaction.

[422flat6]

Installed photos. I think it looks pretty good.

[911 Rod]

Nice job!
Does it get in the way when you look in the rear view mirror?
In my 86 I have to turn my head to see around it. PITA at the track.

[Tremelune]

Grab a 9V battery and some alligator clips...