Right, thanks to RichRobby for that link, I think I may have done it?
I've used 'EveryCircuit' to simulate the circuit, and it works on there. A switched negative input as a trigger activated a 12v output. In this case, when the ground of the courtesy lights has a current, the 12v feed to the side lights will work.
In the diagram, the courtesy lights is just a switch on a ground feed into pin 30 of the relay. The switch basically mimics the courtesy lights being switched on, and the ground feed having a current.
Also, 87b was meant to say 87a sorry - typo.
Off:
On:
So it works in EveryCircuit, and that's going by the instructions on the link that RichRobby posted. Seems a simlply way to convert a negative switched input to a 12v switched output.
Here's my live demo of it:
http://everycircuit.com/circuit/5364238130610176
Have a go. The bulb at the top is the parking lights. The bulb at the bottom is the courtesy light. The bottom switch is the courtesy light switch and the top switch is the parking light switch. If you switch on the top switch in the demo, you'll see the parking lights turn on, but if you turn on the courtesy light, they both turn on.
So I suppose the only thing I ask now is, why wouldn't this work?
It's basically this:
