Hello Got a good one for you. I was driving my 04 F150 FX4. parked in a parking lot engine still running, lost the dash and the heater went out. Engine still running fine. I shut it off and of course no nothing only light that comes on is the Airbag light but no gauges no nothing when trying to start. Towed it to 2 places so far. Now i have it at home and Im messing with it. Sometimes there is a clicking under the dash the 6 prong Flasher relay. I did replace the battery. Now check this out. I was seeing what else did and did not work. The windows no power. The door lock switch would unlock the doors but not re-lock them. Now here comes the good part. While I was holding down the Drivers side door lock switch to the lock position, all the power to the dash and heater came back on!!!!!!! and as long as I hold down the door lock switch the vehicle starts right up!!!!!!. Now once I release the door lock switch the engine still runs. But i lose the dash and the heater again. and I have to still shut the truck off with the key.
Very strange. Why and what would the door lock switch have to do with any of that. and I don't want to drive down the street having to hold the door lock switch down all the time lol. This is a weird one. does anyone have any advice.

 

Multiple seemingly unrelated electrical issues are very often a ground problem. I would start by checking the grounds.

 

Ok. I'll keep looking for the grounds. Would you happen to have a layout as yo where all the grounds are located

 

I don't have a layout, but one that is commonly a problem is under the kick panel in front of the front doors.

 

Do this first. You'll likely find body rot which has become too common on this generation truck.

 

Here's a photo of the three grounds on the passenger side of my 2004 Lariat after removing the plastic covers. The one on the driver's side is in a similar location.

Are you locking and unlocking with a remote control? If so you might want to wiggle the black rubber wire boot between the driver's door and the body to see if it affects the problems.

 

His symptoms are consistent with a faulty ground. Let's see if that idea pans out first.

 

Holding the door lock switch may be creating an alternate path to ground for some of the circuits.