Left rear turn signal doesn't work when running lights are on: Open circuit ground wire?

[myf16]

My 2009 Edge has a fast blinking left signal from the front turn lamp only whenever the parking lights or headlights are on. With the parking lights and headlights off, the left rear turn signal operates normally.

 

I infer that the stop/turn signal circuit is grounding through the running lamp filament, which can only happen when the running lamp is off. Also that the ground wire for the tail lamp socket is not connected to ground. However I don't understand how the running lamp can ground itself through the stop/turn filament.

 

Anyway, am I correct that I need to find the break in the ground connection? 

 

A second possibility is that the ground wire is marginal: sufficient to carry the current of one filament but not two.

 

Edit: Someone had replaced the body-side connector to the left stop/turn lamp. They used a mechanically compatible pigtail connector whose green wire was NOT on the correct pin for ground, but they wired it to ground. I will switch two wires on the pigtail and that should fix it.  

 

Thanks so much to @Haz for the detailed information in the other post from a member with what could be the same problem: 

 

 

Edited August 8, 2022 by myf16

[myf16]

I ended up having to switch all 3 wires. It works now. Fortunately I had a working right side connector to identify the roles of the 3 pins. 

 

Photos show the non-Ford connector, incorrect wire pairing, corrected wire pairing, and waterproof wire nuts before reassembly.

 

The person who installed the connector checked that the signals and running lights worked, but must not have tested them together. 

 

[enigma-2]

I *hate* wire nuts. Especially in a car.

 

In the future, I'd recommend using heat shrink connectors with built-in solder. You slip in stripped wire each side and use heat gun. Solder melt first, ensuring a good mechanical and electrical bond, followed by heat shrink, shrinking down to enclose connection. Good, easy, fast permanent electrical connection you never need to worry about.

 

(If you don't have a heat gun, a propane cigarette lighter works just fine).

 

haisstronica 120PCS Solder Seal Wire Connectors-Marine Grade Heat Shrink Wire Connectors-Heat Shrink Butt Connectors-Butt Splice Wire Connectors for Stereo,Electrical with Corrosion and Weatherproof https://a.co/d/8LTwntd

Edited August 22, 2022 by enigma-2

[myf16]

I had only Home Depot stock to choose from, at 5 minutes until closing time. Beast would have been gel-filled scotchlok connectors. Gel-filled wire nuts should give a permanent repair.