The left rear blinker on my F250 started "fast blinking" today.... It still clicks inside the truck, but the blinker does not work outside. The dash indicator does blink but also very fast. I checked power at the bulb connector and my test light blinked fast too... I'm thinking a ground issue with that connector but am really just guessing.

Any other ideas or suggestions will be greatly appreciated.

Thanks!

I'm thinking Lighting Control Module.

What year is it?

Also remove the leftside turn lights and check OHMs, maybe a light is shorted internally or very low resistance.

Should have said that if the rear is blinking the problem will probably be in the front. Don't forget to check the side marks if any.

You have a bulb out, the F250 does not have a lighting control module. Also while the codition is occuring, try wiggling the hazard button on top of the steering column. This switch will get gunk in it and cause the concern. If all bulbs are ok, you may have to replace the multifunction switch. Ford SMT

Check for a short in the trailer connector.

Had the same problem on my Ranger, its a bad bulb.