Originally Posted by jon@work

Take the original black/red wire off the pump and use this to switch a new relay mounted in the boot (relay pin 85). Run a new wire from the relay (pin 86) to earth. Run a new fused wire (fuse and wire rated at 30A) to the new relay pin 30. From pin 87 run a new 30A-rated wire to the pump, and a new wire from the pump to earth.

When complete, check that you've got over 12V at the pump.