Technical 500x taillight - new connector but now works incorrectly lol

[pocarisweat]

So I'm helping a friend with her 2016 500X. The left rear tail light stopped working entirely, malfunction on dash. I isolated it to a barely melted wiring harness plug to the tail light assembly. This seems common. I ordered a replacement plug with pigtail wires. Pretty sure I got the correct one, even though this Amazon listing is for a Mini. It definitely fits. Checked, double checked, triple checked the wiring order. I'm using waterproof quick connects (with the solder inside) to the body harness.

https://www.amazon.com/ALLMOST-Harness-Compatible-TailLight-63217361305/dp/B08HQCT8R1

After re-wiring, the lights give strange behavior such as the reverse light coming on when the brakes are pressed. The turn signal activates the tail light but not the correct light. I connected the left tail lamp assembly to the right side and it works properly. So I re-did all of the quick connect connections, and exact same behavior.

At this point I'm kind of at a loss. Prior to the re-wiring I was able to get the left tail light to work once by disconnecting, cleaning and reconnecting it, but it failed again after a few days. I'm pretty sure the original problem is the melted wiring harness plug. Now that I've replaced the harness plug, I'm not sure what's causing the new semi-working but not correctly-working tail light behavior. At least her turn signal works again! lol

At this point, I'm considering ordering a new plug on the possible assumption the ground wiring is weak on the one I received. That feels like a lot of hopium though.

Any guidance from a Fiat tail light expert would be much appreciated.

 

[s130]

Form what you have written it sounds like some wires are crossed.

I installed the tow bar and wiring on my UK 2015 500X and thisi is the wiring info for my vehicle. (Rocco is my cars name )
The ZR2500 is a caravan/trailer control unit so not relevant for your purposes.
The RHS Side Light -- Rocco Yellow/Black is the rear right light wiring loom and the associated Yellow/Black wire colour.

I'm hoping that the US 500X models follow the same cable harness wire colouring.

RHS Side Light -- Rocco Yellow/Black --> ZR2500 Brown Wire
RHS Fog Light -- Rocco Brown/Yellow --> ZR2500 Blue Wire
RHS Indicator Light -- Rocco Blue/Black --> ZR2500 Green Wire
RHS Brake Lignt -- Rocco Green/Yellow --> ZR2500 Red Wire
RHS Earth Light -- Rocco Black --> ZR2500

LHS Side Light -- Rocco Yellow/Red --> ZR2500 Black Wire
LHS Reversing Light -- Rocco Purple/Green --> ZR2500 White Wire
LHS Indicator Light -- Rocco Blue/White --> ZR2500 Yellow Wire
LHS Brake Light -- Rocco Green/White --> ZR2500 no connection
LHS Earth Light -- Rocco Black --> ZR2500 no connection

 

[The Panda Nut]

I tend to agree with s130.

I did inadvertently plug the RHS light unit into the LHS plug while doing a towbar once (vW) for testing, and the car knew and threw an error message out so who knows. The fact it works OK may suggest the plugs are handed and all is not quite as you want. I would check all lights and understand what is not as it should be then you may need to swap wires over. The voltages and currents involved in these can bus lights are no longer straight forward. If realigning the wires does not work, and checking the earth very carefully doesnt either you may need a specific Fiat plug and tails from a breakers.
I always solder wiring connections now to reduce any resitance issues that may affect can bus, but have never found things were not wrong due to anything other than crossed wires by me. Lable each one and if necessary test so you know what wire is responding to which switch. Sometimes you just cant see for looking and labelling may help spot your error.