Refer to the 'white wire' recommendation above; but the wiring harness link is to a '05, and it may be a little different on a '07. Per the wiring diagram for '05, the high beams feed off the same relay, both sides; those are working ok for you, which indicates the grounds to both bulbs are also OK (the grounds are through the BEC for '05).
The 'white' wire runs direct to the SJB, and power to the left low-beam is supplied through the SJB; the right side is fed though separate light-green/white wire. Nothing that would indicate a headlamp switch issue.

There are three 30A fuses that feed the headlamp functionality, and those are in the BEC.

 

So...problem resolved. Somehow the computer reprogrammed itself the last time I had a light go out and wasn't feeding constant power to the drivers side low beam. Took half an hour at the dealership once I gave them all the things I've tried.

Thank you all for your help. Without ruling out the suggestions from each of you I'd likely never have figured this out...and it would have cost a lot more at the dealership to have them find the gremlin.
WORD OF ADVICE: Call ford! The manufacturer was surprisingly helpful in narrowing the cause and saving me money!

WARNING! Do not alter ANYTHING having to do with the headlights (or any computer controlled circuit!) as something as simple as the wattage of a bulb (this includes LEDs) can alter the program and either make things not work or burn out circuits!

 

Yea I have a 95 mustang gt and driver side low beam won't come on checked bulb not the problem just.put aftermarket headlights in and.same.problem starting to think its in the plug or a wire any suggestions?

 

Call ford huh? Lol

 

i had very similar symptoms to the original post on my 08 mustang GT. it was tricky to find as i had power across all the white wire of the headlight but an intermitent connection to the little loom that connects into the head light. it even showed continuity right up to the headlight bulb blug on a multi metre but wouldnt get power on a pencil tester. worth a look if you are in a similar boat. i hope this might save someone a whole lot of time as a nother possibler cause. have put a picture together to show what i mean.. hopefully i can attach it here all good.

Gavin

 

i've got the same problem with my 2006 GT except my problem is with the passenger low beam. High beams work fine on driver and passenger sides but low beam on passenger side will shine for approximately 10 seconds and will shut off. i can consistently reproduce this problem, does it every time. i've tried new bulbs in passenger side only, new bulbs in BOTH sides, and tried replacing the passenger side plug, but low beam still cuts off after 10 seconds, even when parked with engine off. so is it a fuse problem, a relay problem or a switch problem? i'm getting tired of running driving lights or having a "three-eyed" pony car. thanks for all reads and replies!