Jerry
Dork
4/18/14 6:23 p.m.
I bought some decent backup bulbs from VLEDS http://www.vleds.com/bulb-type/reverse-back-up/921-led/5k-white-15-led-high-power-bulbs-917-918-920-921-922-923-579-1-pair.html 
And generic Amazon bulbs for the tail lights 
The back up lights didn't light up at all, and the tail lights gave an error on the dashboard as soon as I hit the brakes (I didn't even know there was a warning light). A little Google-Fu sounds like I need the error-free CANBUS bulbs for the brake lights, and I don't know why the backup one's didn't work. Kinda soured on LED's for the back at this point, waste of time. At least they were easy to get to, vs the front.
DrBoost
PowerDork
4/18/14 7:24 p.m.
The BCM is seeing the wrong resistance and thinks there's a blown bulb. Not sure why the reverse lights aren't lighting though.
Just FYI, I have had terrible luck with LED taillights. They are basically invisible during the daytime, I've gone back to incandescents every time for fear of safety. Also, did you try the backup lights both ways? LEDs are polarity-specific, you might need to flip them in the socket if they don't light up.
Yup, flip those reverse lights around.
I've got 100% LED lighting on the exterior of one of my Miatas. Even the headlights. No eBay stuff, it's all from Superbrightleds and it wasn't cheap. But everything is as bright or brighter than stock and thanks to a resistor change inside my flasher module, they don't hyperflash. I'm really happy with them.
For my reverse lights in particular, I chose a more directional light. They work beautifully - but for those, I had to move the two wires in the harness to get them the right polarity.
Jerry
Dork
4/18/14 8:06 p.m.
Keith Tanner wrote:
Yup, flip those reverse lights around.
Tried that, also swapped bulbs and flipped again (by myself and could only see the driver side, with the assembly hanging over the fender). I had LED backup bulbs in the xB without issue. Well, except for when I bought both sets of bulbs, they lit up fine, but then didn't fit in the housings for being too berkeleying long.
You could easily check your reverse bulbs with a piece of wire. It's not complex stuff.
Jerry
Dork
4/19/14 6:46 a.m.
In reply to Keith Tanner:
I forgot I made a long test rig with two wires and alligator clips on each end, back when I thought I was a tuner kid and wanted LED's everywhere in the xB. Maybe I'll 2x check today. (Pretty sure I'm returning and investigating PIAA lights)
Keith Tanner wrote:
Yup, flip those reverse lights around.
I've got 100% LED lighting on the exterior of one of my Miatas. Even the headlights. No eBay stuff, it's all from Superbrightleds and it wasn't cheap. But everything is as bright or brighter than stock and thanks to a resistor change inside my flasher module, they don't hyperflash. I'm really happy with them.
For my reverse lights in particular, I chose a more directional light. They work beautifully - but for those, I had to move the two wires in the harness to get them the right polarity.
You can't really cheap out on LEDs yet. Gotta get the good stuff.
Yes you have to get the good stuff but I've had just as many issues with superbrightleds.com as I've had with amazon ones.
pirate
Reader
4/19/14 6:38 p.m.
I have LED tailight/turn signal/brake lights at the back of the car and LED turn signals at the front and nothing would work until I installed a flasher that applied a load to the electrical curcuit. I purchased the flasher from superbrightleds.com My tail lights/turn signal/brake lights are very bright
SlickDizzy wrote:
Just FYI, I have had terrible luck with LED taillights. They are basically invisible during the daytime, I've gone back to incandescents every time for fear of safety.
Did you use clear or red LEDs? From what I've read the white ones act as you described and the red ones work great.
Yup, match the led to the lens color. Otherwise a bunch of the light ends up being filtered out.
In reply to Junkyard_Dog:
I used red, but they were eBay cheapies and the housings on my old Jetta may not have been the best design. Maybe I will try again with some better LEDs.
I'll get some pics of how mine looks vs incandescents when I get a chance.
Jerry
Dork
4/19/14 9:47 p.m.
I've got PIAA bulbs lined up for fog, DRL, and backup lights at this point. Other Fiat owners attest they work very well.