Yes just like that but with a 30 amp fuse between battery and relay.

_Stingrey_ said:

Yes just like that but with a 30 amp fuse between battery and relay.

Click to expand...

Good.
Have you worked out the current draw to determine that you 'needed' a 30A fuse, or was that just waht you had ?

Anyway, ignoring all that ...

To make the lamp illuminate (in my drawing) you put the relays coil to ground (with the switch) which energises it and the battery power is fed from 30 to 87.
As you want the lamp to light when another one does then you would put 12V (from the other lamp) to 86 on the relay (in my diagram) in place of the thin red wire...

Does this help ?
In this pic the Dash Mounted Light Switch is substituted by the 12V from the High beam ...

miktdish said:

Have you worked out the current draw to determine that you 'needed' a 30A fuse, or was that just waht you had ?

Click to expand...

Very valid point above ^^
2x 100watt halogen spotlights have a 16.6 amp draw at 12v so you do not want more than a 20amp fuse.
If you are using a standard 2x 55watt bulb then that is a 9.2amp draw at 12v so you want a 10 amp fuse.
If you are using LED then they are likely to need a very low powered fuse. I have 5x led spotlights on my bulbar running off a single 10amp fuse.