For the short term, I would take the knob off the faucet as suggested and check the contacts. Polish/clean the contacts, make sure there are no breaks in the wires. Due to the location by itself, those wires are prone to corrosion. I finally got disgusted with mine working sporadically, and ordered one of the surflo ones. It's the same type that are used in the later model Euro vans, and a much nicer faucet in general. You can throttle the output, whereas the stock one goes ful bore, and I require it making a mess. Of course, the pumps only last so long too. And as mentioned, check the fuse on the B piller behind the driver seat. This is another area that can sometimes get corroded, so taking a little bit of steel wool to the fuse and contact may help there even if the fuse is good.

Good luck,

Troy