An old phone charger? I don't know if that'll be enough to power the washer pump. What is the amperage on the charger and pump? If you try to draw too much power out of somethin that's not made to give it, you'll have a fire on your hands.
Ex. My cell phone charger pushes a full 0.7 Amps at 5.1 V. That's really a small amount. Also, is the phone charger 12 V? You could always go to your local electronics store and pick up a DC
power source. When I was at radioshack recently I saw a 12 V, 1 or 1.5 amp power source for $25 bucks maybe? Or you could always make one from an old PC
power supply. Those'll give you like 10-20 amps or somethin depending on the one you have.
They're really easy to make too, just go to Instructables - Make, How To, and DIY
and type in ATX bench power supply. Much like this one... ATX to Lab Bench Power Supply Conversion
Took me a full 10 minutes to make mine...