When I turn the left blinker switch on, its blinks fast and the left rear doesn't blink. I bought a new bulb, still doesn't work. I did a search and read a lot last night and thought I could figure this out- naught. What I have done:
1. Ohm'ed the ground wire (black) where the rear bulb goes and it came out 69 with the ohm meter set at 2000.
2. Ohm'ed the multifunction switch, checked out fine.
3. My Chilton repair book said the hot wire to the L. rear bulb should be LG/OG; mine is BK/pink(red). It does appear that the LG/OG is correct at the multifunction switch but it changes somewhere along the line to BK/pink. At any rate I took a 8' piece of telephone wire and ran it from the L. rear light (BK/pink) up to the cab and place my ohm meter between it and the LG/OG. It ohm'ed out at 96 with meter set at 2000.

My question is is that too much resistance in either the ground or the hot wire? Does anyone know where the junction is that they change wire colors?
Any other help please>

 

Was your Multifunction Switch replaced in January - May 2011 with an OEM Part? If so, its recalled for this issue. Compliance recall 11C18 OEM Motorcraft Multifunction Switch. Attached is what I have for your vehicle wiring schematic.

 

Multifunction switch was not replaced, maybe the problem. Thanks

 

Any interruption in the juice to the signal lamp will cause the flasher to speed up. So if replacing the switch doesn't fix it, locate the ground wire connection from the socket to the bed and un-bolt that connection.
Sand the frame clean of rust, apply small amount of grease, inspect crimped connection on the lug before re-bolting.

69 ohms is way too high for a good ground from the socket to the bed.

There is a wire harness plug near the rear of the bed that can be pulled apart to check for flasher voltage with a volt meter.

 

Replaced the multifunction switch, no change. Will look for the ground and clean it up and see what happens. Thanks retiredsparky.

 

Blinker fluid low?

 

Mine did that with a new bulb also. I then cleaned the socket, reinstalled and all was and is well. Just a thought.

 

Cleaned the ground and the ohm's reading went to 9 from 69 but still no brake light or blinker on left. I did note that when I put it in reverse, the light that the blinker uses came on. This suggests to me the problem is where the turn signal and brake light have something in common but I don't know what or where to look. Company coming tonight so will see if I can get blinker voltage somewhere. Any other thoughts?

 

Retiredsparky: Had a few minutes and went out and checked the flasher voltage at the connector that is up and behind the spare tire. I had flasher voltage there which is about 3' from the turn signal bulb. I started following the wires up towards the bulb and right where it makes a 90 degree turn going upwards toward the bulb, the hot wire was warn through. I put a jumper wire to make sure that was the problem and low and behold, blinker lights!!!

Thanks everyone for the help.

 

Glad you got it.