If only wanting to check the pump voltage use a digi metre and speaker wire, this is thin enough to be inserted into the plugs on the connectors, trail wire in car through rear window etc, hook up to metre and note reading under FULL LOAD.
If only hooking it on live it will give a spurious reading, needs the same earth as well to get 100% acurate reading, ie pump earth, easier job with digi metre then just rewire afterwards and note the change.
tabetha