Good Morning Everyone,
2006 F150 King Ranch 5.4L, no modifications that I am aware of. My issue is, when my headlights and/or running lights are in the on position;
1. The left turn signal stays lit, does not blink.
2. The right turn signal works as normal
3. The brake lights, including the high mount brake light, act as if the pedal is engaged
4. CC does not turn on
5. When I use the left signal, the door locks engage, do not engage while driving like normal

Again, all this only happens when the headlights/running lights are ON. Does not matter if it is the auto lights or actually engaging the headlight switch. When the headlights/running lights are OFF, everything works as it should.
The only things I have done so far is try a new flasher relay (did not help), changed the whole high mount light assembly (non LED, I thought that may have been the issue as there was some melted sockets on the original).
I checked the all the bulbs for reversal and/or shorts to ground as well as the trailer electrical connections. No fuses out.
I am a pretty handy guy and do all of my own services and I cannot for the life of me figure this one out.
It all happened out of nowhere one day. Driving with the cruise on (it was raining) and the cruise cut out like I had engaged the brake. Reset and tried again, did the same thing about another 4 miles down the road. Did not notice the turn signals until the next morning driving to work.
I appreciate any help y'all can send my way.
Thank you and stay safe!

 

I'd start with checking your emergency brake pedal - make sure it's releasing and staying all the way up.

Next check the brake light switch. May need replacing. Very easy swap.

 

I would pull the third brake light out and inspect... water and corrosion hot spot.. and the third brake light controls the cruise control circuit...

 

Roger that. I will give those a whirl and report back the findings.
Thank you both!