Do all of the above regards checking the earths and if necessary fit a secondary earth in too, it has been noted that there can be high resistance coming through the loom to the lights too on older models.
To get over this you can do as I have done and fitted relays in to the lighting system, there are a number of ways to do this.
you can take either of the lamp wires and make a new loom using the old lighting wires as trigger wires to the new relays and then feed both lights from the new relays.
you can fit relays on each side and power the lamps individually from each relay.
There is another option that I don't understand so won't embarrass myself trying to explain it.
Along with running something like OSRAM Night Breaker bulbs you will have great lights.