Quick update:

I switched out the taillight/brake bulb and front turn signal bulb on the left side (the problem side), and cleaned the ground connection for the left taillight. This is all I've had time for recently. Unfortunately it hasn't improved anything. I guess I'll clean the other taillight ground. I couldn't replace that bulb because it's stuck in there and I don't want broken glass embedded in my hand from pushing any harder.s

 

Use gloves to get it out, or replace the socket, (probably a better option) It might be stuck in there due to someone forceing it in wrong.

 

The bulb that is stuck is on the passenger side (the side I'm not having an issue with). Could that be causing a problem, or is that unlikely (still trying to grasp how the electrical is working).

 

1975/79 F100/350 Styleside: The socket is plastic, separate from the plastic taillamp body, twists into the body.

The same sockets, same taillamps were used on 1975/91 Econolines.

 

Update:

Checked for continuity between the chassis and both taillights, reverse lights and the front left turn signal.--Is it normal to find "continuity" on every wire of the taillight & turn signal harnesses? Because I did.--Inside the taillight socket I found continuity between the chassis and both contacts at the bottom of the socket (same on the front turn signal, if I recall correctly). Continuity between the chassis and both contacts on the left reverse light and only one (side contact I think) contact on the right reverse light.

Does any of that mean anything? Narrow it down or point in any direction. I'm confused about the continuity that was found on every wire in the harness for most of the lights.

It was mentioned earlier that there may be a "wiring fault between the parking lamp circuit and turn signal lamp." How do I check that?

 

No it's not normal to find continuity on every wire. That is where your problem probably lies. You should only have continuity from ground to the side contact in the socket with the bulbs removed. I could understand if the bulbs was still in the sockets however.

This is what should have happened.

Left rear turn and brake socket continuity test.

1: Continuity between Chassis and side connection.
2: No continuity between chassis and the two bottom connections, with the bulbs removed.
3: No continuity between the two bottom connections.

Check the continuity with a multimeter set to ohms, and all power to the truck off. Disconnect the battery ground cable etc...

Check for continuity between the two bottom connections. One is for the turn and brake, and the other is for the parking lamps (taillights). If you got continuity then they are crossed somehow. Yes the right side could affect the left as some of the wires for the right are also on the left, so change that socket.

 

Thanks, 81-F-150-Explorer!

On the front turn signal, which contact should I find continuity with the chassis?

I hadn't disconnected the negative battery cable when I performed these tests. Could that have skewed my results somehow?

 

The Front is the same way. Check ground continuity from the chassis to the side connection.

The battery being connected could skew your results if power was going through the wires at the same time as the test. (You never know)

 

You just nailed the problem. You have a classic symptom of a bad ground at/to the left bulb/socket. The turn/brake filament and the park lamp filament in the bulb share the same ground. When you lose the ground, the park lamp seeks a ground back through the turn signal indicator bulb.
Check for continuity between the body of the truck to the ground pin in the socket. You can't see it, but the ground pin corrodes down inside the shell of the socket so there is no connection between the pin and the wire.

 

Chassis Continuity Update:

Left taillight:Left reverse light (does not work with parking lights off or on):Left front turn signal:Right taillight:Right reverse light (does not work with parking lights off or on):Right front turn signal:Side-marker lights (all work fine):None of this makes any sense to me. I was hoping that something would stand out as a clear oddball, but it's all random (to my novice eyes). I was very thorough and took a great deal of time double and triple checking these results. I've cleaned both taillight grounds. I haven't had an opportunity to clean the battery/engine or engine chassis grounds.

Additionally (since testing all of the bulbs & harnesses), the left turn signal is now a little slower than normal without the lights on. The signal now flashes (without parking lights on) in the driver's taillight and not the front driver's side light. With the parking lights on it flashes in the front driver's side light but not the taillight (it goes black).

I've also noticed that when the parking lights are on and the left turn signal doesn't work (just stays lit on the dash when turned on...sometimes works sluggishly) that if I press down harder on the turn signal lever that the turn signal indicator on the dash goes out.

What's going on?!

 

the lights flashing opposite each other is ussually a indicator of a bad ground.

 

Replace the darn left rear socket and be done with it already!!

 

Actually I'd replace both rear sockets and the front left one as well.

They all seem to be bad in one way or another.

 

I'll do that. So frustrating/confusing, this electricity. I'll get those ordered.

Thanks!

 

Haven't received the sockets yet, but noticed today that now the front left turn signal/hazzard isn't working. The parking light works, but if the turn signal or hazzard is on it goes black. Related to any of this??? I have no idea.