Alright, this has both me and my dad completely stumped. I own a 1985 Ford F150, single cab long bed. It's a 5.0L 302ci v8 FUEL INJECTED motor. This was the very first year/model to have a fuel injection system on it. So naturally there are wiring issues.

When my headlights are on, neither the break lights will come on, nor will any of my turn signals front and back. When my head lights are off, all signals and break lights work. Also, when my headlights are on, my back running lights won't come on now, they used to, but not now. We've replaced the turn signal switch, as well as the connector to it. We have a brand new light switch as well. All the bulb connectors are good, our grounds are grounded, and we've even switched out the little canisters the blinkers use. An interesting note, sometimes when the lights are on, we can get one little blink out of the blinkers on the front. It's acting like the lights are a drain in a way, when the lights are on. This is a very very strange problem. We've already pulled the entire dash and are examining every wire closely, but it's hard because the previous owner did a very bad chop shop job on the wiring harness when they changed things out. =/

So far we have almost everything working, and this is the last bit is getting my lights in place. We're thinking it MIGHT be the fuse box. Some weak connection there that's not letting the lights blink when the lights are on. But that doesn't quite explain why my back running lights aren't coming on now, or why my break lights don't work when the lights are on. Any ideas? We're really beating our heads into a wall here trying to figure this one out.

 

I would pull the bulbs front and rear and check with a meter for a ground in each bulb connector. This is with both the headlamps and the parking / turn lamps.

I only see the comment about "our grounds are grounded" but it does not specify if you pulled both the headlamps and the park / turn bulbs, and checked those separately.

What I got from your post

1. With the main headlamp switch in the off position
1.1. Front Turn signals work correctly
1.2. Rear Turn Signals works correctly?? ( not 100% sure on this one )
Q1 -> Is this true for both the front and rear lamps ?

2. With the main headlamp switch in the park position ( parking lamps only on headlamps off ) the result is ?
2.1. Front Turn signals
2.1.1. Front Parking lamps
2.2. Rear Turn Signals
2.2.1. Rear Parking lamps
Q2 -> What happens in this state for the front and rear turn signals, as well as the parking lamps

3. With the main headlamp switch in the head position ( headlamps and parking lamps on )
3.1. Front Turn signals
3.1.1. Front Parking lamps
3.2. Rear Turn Signals = Do not work
3.2.1. Rear Parking lamps = Do not work
Q3 -> What happens in this state for the front turn signals, as well as the parking lamps, confirm I marked down the rear correctly.

Just want to clarify what is going on, it really does sound like a missing ground, and that is why I asked what happens when you have the parking lamps on ( headlamps off )

If I am not mistaken, this MY truck does not have the CHMSL ( Center High Mount Stop Lamp / 3rd brake lamp ) ??? Can you please confirm this.

 

headlights are lowest resistance so they get power first. Sounds like you have a power or ground issue.

I would also check for ground straps from the cab to the frame.

 

Sounds like an open/poor ground for the turn signals and brake lamps.

 

does/did the truck have a 4 prong trailer harness?

if the running light circuit is shorted out...(this would explain why theres only problems when u turn the truck running lights on)