Lt headlight wont work in low beam

find the low beam light relays
by law the fog lights can only come on with low beam
using fog lights on high beam is *******

you have a double headlight has low beam and hI beam and the low beam has gone cuz its probably use the most so replace them boththat way you will have both of them look in the same best place to get them would probably be Amazon they have very good prices on them

on the 2005 it was on the left side of steering column low on the dash panel.

check low beam fuse when switched on if it has voltage registering across the fuse ,if there is voltage across the low beam RHS fuse then it is possibly a pinched wire from the headlight running back towards inside of the car

there is a fuse under the hood for left low beam light and right low beam lights

Cavity/ Fuse /Circuits
1/ 20 Amp Yellow/ Left High Beam Headlight
2/ 20 Amp Yellow/ Right High Beam Headlight
3/ 15 Amp Dk. Blue/ Adjustable Pedal
4/ 20 Amp Yellow/ Horn
5/ 25 Amp Clear/ Headlamp Washer - If Equipped
6/ 15 Amp Dk. Blue/ Front Control Module (FCM)
7/ 20 Amp Yellow/ Fog Lamp
8/ 15 Amp Dk. Blue/ Park Lamp
9/ 15 Amp Dk. Blue/ Non ABS Brakes
10/ 5 Amp Orange/ Starter
11/ 20 Amp Yellow/ Auto Shutdown/Powertrain Control Module (PCM)
12/ - -
13/ - -
14/ 25 Amp Clear/ Powertrain Control Module
15/ 20 Amp Yellow/ Injectors, Ignition Coils
16/ 15 Amp Dk. Blue/ Powertrain Control Module
17/ 30 Amp Pink/ Antilock Brake System (ABS) Valves
18/ 30 Amp Pink/ Windshield Wiper/Washer
19/ 50 Amp Red/ Radiator Fan
20/ 20 Amp Lt. Blue/ Starter
21/ 50 Amp Red/ ABS Pump Motor
22/ 40 Amp Green/ Radiator Fan High/Low
23/ 50 Amp Red/ High Intensity Lighting
24/ - -
25/ 30 Amp Pink/ Lighting Left High Beam/Right Low Beam
26/ 20 Amp Lt. Blue/ Transmission
27/ 30 Amp Pink/ Lighting Left Low Beam/Right High Beam
if this helps please rate 4 thumbs up.

Replace Relay #46 in underhood fuse panel.

Did you try the hi beams ? If they work, could youpossibly have 2 burnt out low beam bulbs.? I'm a pro, but it faked me out one time.

Take a look at this link regarding DRL system. I know it's not exact truck but most of it would apply to yours too. It should give you hints on what to test & how.
http://www.fordf150.net/howto/drl.php

First place to start is with headlight switch. I did have a case many years ago when both high beams were blown out at once, but this is rare. When you have the headlight switch out, jump power from te low beams to the high beam contacts. If high beams come on, replace switch.
Bill