The taillights, brake lights and turn signals on my 03 F150 seem to be getting weaker and dimmer over time and I'm beginning to suspect a bad ground on the bed. Is this a possiblity and does anyone know where the ground strap is? Thanks

 

The diagram shows the rear ground as by the left front radiator support / driver's side headlamp area.

1. did you check the voltage at the bulb connector to see what the value is ?
1.1. Compare this to the trailer tow adapter stop / turn & parking lamp circuits.
1.2. Use the trailer tow circuit and the truck's bulb connector ground and compare it to using the trailer tow adapter ground, which is grounded to the frame of the truck.

The bulbs could just be on the way out, getting dimer with age ( or it is a subjective comparison not what is really going on ), but you should confirm the VDC values before swapping parts.

 

OK, I'll run voltage checks on those circuits and see what the difference, if any, there is and also replace the bulbs. They are the originals. The bed ground is up front? Strange location ! Thank you, I appreciate the response and help.

 

I think there is another ground under the drivers door. Ive seen a cable there when changing the fuel filter.

 

No the ground for the stop / turn lamps is marked as up front.
- The bed is made for hauling stuff, not a ground plane.