Do a search on here, but roughly you use the original pump feed(live) to trigger a relay to supply the pump.
You will need to run a new live supply wire from battery(through fuse at battery) to the rear of the car, I mount mine just abovethe rear n/s suspension turret, there is a hole in the metal work there which is 8mm so is ideal to mount a relay holder.
I also run a new earth to the pump/relay.
As wiring looms age they build up resistance, so even measuring 12v at the pump can drop considerably under load, as the digimetre does not take any amoun of current, where as a pump does, 18 amps at lockout on a 044.
If you haven't done the pump rewire, I suggest you also look at fan rewire.
tabetha