1999 F150 4x2 extended cab 248k miles. New to us, and I am struggling with electrical gremlins.

No right turn/brake light, tail light works fine. Rapid dash indicator light. Left side works fine. Replaced bulb/socket, no change. Inspected wiring all the way up to the first major plug above the axle. Checked for power at the plug, the 7 pin and the 4 pin trailer plugs, nothing. A little internet research suggested the multi function switch, so swapped it out. Still nothing. Noted a loose wire at one of the harness connections, looks like O/LBL which should be our culprit, but stuffing the multimeter probe into that connection still shows no joy. Same test on the left turn/stop wire shows power. So we are hoping someone can identify our problem before I start rewiring the truck and/or replacing every switch/relay.

Thoughts?

 

Your test results as stated indicates the MFS is bad, no matter how old it is.