Have you looked at the Workshop Manual... Mind you, don't trust it 100%, as it does lie....:lol
I am assuming you do not have "Daytime running Lights", as they add another control box into the equation!
Look here...
http://bebrs.pie-dabas.net/Webs/Terrano/R20/el.pdf
The Head light wiring diagrams 26/292
The Headlamp Aimng control is on 37/292
According to the diagrams the Head light actuator has nothing to do with the actual working of the head lights, and is a completely separate circuit that just raises and lowers them.
Looking at the wiring diagram, there are 2 relays located in the foot well of the car, one on passenger side controls the Head Light that side, and one on drivers side that does that... Look on page 243/292 for a clue to exactly where...
Now, it is a bit hard to follow, but it looks like the relay controls only the dipped beam, and the high beam is direct from the switch, and turns the dipped beam off via the relay when you are on the High beams.
Personally, I would find the relays first, and while touching one, flick from dipped to main and back... you should feel them click... if one does not click, I would swap them over from side to side, and see if the problem moves... If it does, it's probably the relay.. Even if both do click, it might be worth swapping them over, as it could be the contact has burnt out.
If it does not swap sides, or both relays do click, you are more than likely looking at damaged wiring... and for that, you will need to get the multi-meter/test lamp out and see which bits of the circuit are and aren't working.
I think I would also have a good look at the Earthing, (I know you mentioned it already) as the way it is wired, if the earth got damaged, you could have a weird symptom where the Dipped, and Main plus the bulb on the other side can all end up connected in series, giving the effect of only having one beam.
If you have one, go with the Multimeter, or Test lamp (easy to make, just a low wattage bulb on two ling lengths of wire), and see if you have power at the Main beam using a separate earth directly on the body, rather than the one on the socket.