1981 F150 4.9L 300.
When I use the right blinker, the left blinker and side marker flashes with it.
I have already replaced the Turn Signal Switch and the Flasher to see if this would remedy the issue and it has not.

Any ideas?

 

Is the problem in the rear only or in the front turns only or both?

 

I had to go check. They do flash in the rear as well. When the right blinkers are selected, the left blinkers are flashing, but are slightly dimmer as well.

 

It sounds like a bad ground to me.
As an easy check replace the bulbs both front & back on the right side, heck do both side so you know they are good. I have seen the filament short out bulbs and cause some crazy stuff.

If that does not fix it check grounds.
Lets start at the battery.
The stock NEG. cable runs down to the top of the frame rail then on down to 1 of the starter bolts.
That should take care of the frame & motor / trany ground.
Frame ground bolt.

Starter bolt ground.

Then there is a ground wire (8 ga) on the left side of the motor to the fire wall.
That should take care of the body but ........ would not hurt to have a wire from the battery to the inside of the fender.
The last I know of should be from the fire wall to the hood on the right side. (no pictures of them yet)

Now we have a bed way out back that the bolts go down thru wood so I am thinking the bed ground is not so good.
Somewhere I would run a 10 ga (min) wire from frame to the metal part of the bed so you know it has a good ground.
Now we can still have an issue as the lights (should be grounded when bolted) bolt to brackets and the brackets to the metal body. So you can have a bad connection at any one of them points.

Maybe if you can pull the bulbs at the rear and see if the fronts work and if so the issue is out back or do it the other way around would be up front.
Dave ----

 

When it is dark outside, get someone to press the brake pedal while you carefully observe the front blinkers. Are they glowing when the brake is pressed?