There is a good possibility that GM trucks of that vintage had problems with the headlight switch. Check it before you replace it. Remove the 4 screws at the corners of the dasboard assembly, pull the assembly toward you and you should be able to get to the switch.
Possible ground issue at the taillights.
its on the left turn signal switch, at the end is the headlights, then one in is the fog lights, you twist it just like you would the headlights
the dash and radio lights are supposed to dim when the headlights come on. Make sure you have the brightness of the dash lights turned up as much as possible.
A bad ground wire will cause dim headlights, or will cause lights not to work.
probably the fuse is blown.
I would replace the headlight switch as Ford has had their share of these problems.
Switch? both headlights?
A broken wire, a dead battery.
Because it is British.
if you recenly replaced bulbs u may have installed them backwards they should be running lights and brake lights if this is the case they maybe installed incorrectly
Perhaps the high beam part of the lights are rs
it could be the headlight relay. if not it may be in the switch.