hi guys a friend of mine has an st but has developed an issue with the headlight on the o/s turn the lights on and only the nearside lights up but connect up the nearside headlight to the os wiring and it illuminates also if you put the offside headlight on the nearside wiring the headlight lights up bit of a wierd one any sparkys on here possibly have any ideas especially as the wiring checks out

 

has it got hid/ballast fitted-unit might be u/s or the bulb itself

 

Check the earth points underneath the headlights. Common for them to corrode and make the headlights play silly buggers.