Three main causes here.
1) The earths - either between the lights and the wings, or between the wings and the engine (via the rest of the body), or the engine and the battery -ve (although it would be difficult to start if it the engine -> battery was at fault
2) The power supply to the lights - the main causes here I suspect will the the switch, or the relay under the dash to power the main beam circuit
3) The dim/dip relay - located under the dash - it is there to cause the diped beam to come on at 1/2 power when the side lights are on only - it is useless, and causes many problems with the lights as it usualy lasts about 6 months before burning out. - Just unplug the plug out of the side of it - do a search on here for dim dip relay to find loads of pictures of what it looks like
As to work out whats going one here out of the above three issues, I would first get a bit of cable and connect it to the -ve terminal on the battery, then take this to the front of the landy, and then remove one of the bulbs from the housing, and then connect the wire going to the battery to the -ve connector on the bulb - if it becomes brighter then its an earthing problem - look under the wings, have a look at your earthing points onto the wings for corrosion/mud etc and clean/repair as need.
If they stay dim then you need to look into the +ve side of the circuit
I would start by removing the dim dip relay from the circuit to see if this helps. if not then you need to have a look with the aid of a multi meter on volts witht the black wire connected to the -ve on the battery, and follow the circuit from the switch through the fuse box etc to the lamp and see if the voltage is decent (i.e. over 12 volts)
This sort of problem is difficult to diagnose over a forum as there are so many causes of this sort of behaviour, but i hope this gets you started
