2004 F150 Heritage Supercab, manual heater/AC. Turn signals stop if parking or headlights are turned on. Also if heater fan is turned on. I had the blower resistor problem where the blower only worked on high speed. Then turn signals stopped working. Upon researching the TS problem I found the fix to the blower resistor. Putting the new resistor in caused the TS to fail 100%. I put the failed resistor back in and get TS to work unless I use the headlights or parking lights. The blower stopped working at the same time the TS problem started. I have verified that all fuses and relays are working. If all the manual heater controls and blower are disconnected, the TS still fail when headlights/parking are used.

 

You have a bad ground.

Remove the right front passenger footwell kickpanel and inspect the grounding studs. Bet one (or both) are either loose or corroded.

 

I am not familiar with "grounding studs". I am now, after the ordeal of this, intimately familiar with the fuse panel...

 

I see them now. They look pretty corroded. If you don't hear from me again, you have solved my problem! Thanks so much!!!