Most likely the problem is the grounds of your trailer wiring since the stoplight/blinker circuits require more amperage and that's typically where an insufficient ground problem shows up. Here's how you will test everything though:
First thing you should do at this point is test the 4-way of the wiring harness with a circuit tester part # PTW2993 for voltage on all of the right pins when the taillight signals are being applied. Just like in the video I attached.
If the voltage checks... view full answer...
There should be a push tab on the power plug going to the mirror on the CIPA Auto-Dimming Rearview Mirror, you can use a small flat head screwdriver to push down the push tab. I've included a picture for you to refer to. view full answer...
The wiring for the CIPA Auto-Dimming Rearview Mirror with Compass, Map Lights and Temperature Display should be plugged directly into the back of the mirror.
You may want to check your end connections that would have been made underneath the dash for a power and ground. The ground wire should be attached to a good metal grounding point, and the power wire is connected to a circuit that is only on with the ignition. view full answer...
The wiring harness used on your 2009 Chevy Impala must not be grounded properly or it is possible that there are bent or broken pins in the harness they used. The ground needs to be attached to clean and corrosion free bare metal surface. There can't be any paint or coating either.
Check the connectors for the harness to see if there are any bent or broken pins. A bent pin might be able to be bent back but if a pin is broken the harness will need to be replaced. I recommend # 118417. I... view full answer...