Electrical problems are always difficult to find and the answer is never what you expect. I wouldn't expect it's the light switch like Jay suggested but it's possible. The light switch is electrically isolated via the two relays from the lights. Power to the motors of the lights gets there via the "circuit breaker" (on a US model 79 tr7) on one wire (the wire is colored brown and pink btw). This forces the lights to operate at the same time...but they're not so somethings wrong after the breaker.
It's my view to always check the easy stuff first. check the wires are tight and not shortting first...(focus on the brown and pink wire from the "circuit breaker" if that checks out ok, next check the black realys. pull one out at a time, check for rust, try cleaning and replace. if you see rust replace them (this may be the problem but before you do that test them...a rusty relay can still work, we know only one relay is bad since you have some function). To test a relay, you'll need a good one first.
Do you have AC on your car? does it work? You can use your AC relay, if your AC works, to test and locate which light relay is bad.
if you have no AC we'll have to find the relay another way.
since your lights function (not correctly but they do) one relay must be good and the other bad and from the drawing we know the #2 relay must be the one that's bad...since you have some function. Pull one relay out and try you lights. what happens? if nothing happens you've found #1 relay (#1 relay will turn off all power to both lights). if one light works but not the other (the other is completely dead)...you've found #2.
Remember, since you have some function...it has to be #2 that's bad.
Now get the starter motor relay...we know it's good because the car starts, right?. In my car it's in the glove box...get the black one that looks like the light relay...they're exactly the same. (don't try starting the car with the starter relay removed).
Now replace the #2 relay that you found early with the starter relay you know is good.
Try the light...they should work...if it's the relay!
If they don't work...it's the F!@#en light switch like Jay said! Have a few drinks and swear a lot at us Americans.
Before you go thru all this...do the easy stuff first...check the wires.
-Vino
