2006 F150 crew cab short bed
Tail light intermittent as are rear marker lights
Is there a common ground point
With bulb removed and using test probe i have power (light) on all three wires
When stepping on brake both filaments light

 

Yeah, you've got a bad ground.

The grounds for ALL the lights on the rear end (stop/turn, parking, license, & reversing) of the truck are spliced together and then run back into the cabin where the ground connection is actually made (left hand A-pillar, probably behind the kick panel by your left foot). On the way from the back of the truck into the cabin, the circuit goes through three connectors. My best suggestion is to crawl under there and inspect every one of those connectors along the left frame rail and be on the lookout for any corroded connections. Pull that kick panel and inspect the grounding stud(s).

Good luck!

 

Having a similar situation myself. I have a 2005 F150 crew cab 5.4L. My problem is a loose connect where the wire harness connecting to the fuse box is not making a solid connection to the fuse box. When I manipulate the wires by hand, my brake and directional lights work fine. That plug, which is the top right plug connection to the fuse box has 6 wires. One is the ground and the other five are the left stop/turn signal (yellow), license plate lights (brown), stop lights(red) , back-up lights (light green) and right stop/turn signal (dark green). In my case, either the plug/socket is bad or the fuse box board is bad. If the ground is ok, you might explore checking to see if current is flowing out of the fuse box to the wiring harness.pins. Just a thought. I am not an electrician

 

Hello... Thanks for getting back to me on this. I have finally had the time to spend troubleshooting my light problem - as you pointed out a good place to start was the connectors. After that the main ground connection - bonding jumpers - all were in good shape. Bringing a wire from battery (-) and bypassing the whole truck I stabbed the ground wire and all lights on rear worked as they should. So it was the ground as suggested earlier. In the end I went along harness doing the same and when I lost the lights went back and found a small crush in harness at the front of wheel well. Three wires were damaged and the broken one was the ground. Must have kicked something up and around into harness.What a puzzle - fixed now. I know the grounding system well now
Thanks again to all for helping out with this.

 

Wow, that's an odd cause of the grounding issues, certainly didn't expect that particular scenario. Good job finding and fixing it!