Wiring a relay to a switched supply

[Wannabay]

I am installing a PMS3 power control unit in my van together with new interior, Smev sink/hob, LED lights, compressor fridge etc
The unit should be switched off when driving but there is a supply to the fridge, should you need it, via a standard 4 pin relay from the starter battery
My question is, what's the easiest accessible switched supply? Do I have to run a cable from the fuse box or is there somewhere I can tap into in the engine bay?
Thanks in advance
Gerry

 

[KateLovesPurple]

Coil positive side will give you 12v with the ignition on.