A relay is simply an electric switch. It only uses around 200ma to open and close its contacts so when you operate say your side lights only a tiny amount of juice is put through your aging switchgear to turn the relay on and off. The main wiring is still used but is opened and closed via the relay.

I use these 40 amp, fused relays but change the fuse to suit the application. The fuse should always be a lower amperage than the cable used so the fuse blows and not your wire.

If a 4 pin relay is used for simple on or off switching then wire them thus;

Make sure you have a look at the back of the fuse board ...
spent a whole weekend repairing a melted Ovlov 440 dash because of a 'blown fuse' ...

You can get a 5 pin relay which has two 87's
http://www.vehicle-wiring-products.eu/product.php/644/12-volt-5-blade-relay

I installed two at RH side of front slam panel under the bonnet used the original lighting supply which has a connection in that vicinity to power relay 86 , ie Blue red to one relay and blue white to other then fed a new supply to each on 30 and then left and right main and dip to each relays 87's

If you're going to have fuses in the circuit then you really should split left and right sides into separate circuits. Land Rover sensibly didn't fuse the head lamp circuit. Fuses tend to blow randomly (or in landrover's crappy holder fall out). If you've ever been tooling down a windy tree lined country road in the middle of a moonless winter night and had all your lights go out you will know that it's a sight more dangerous than a small electrical fire!