The generator itself should be wired to the battery thru a relay. No other source than the battery should be used to power the cell but the battery.
Either install a switch for the relay , that will require drilling holes in the firewall. I power my relay from the windshield wiper motor hot wire thats is only hot when the ignition is on. The problem i see wiring it to the fuel pump relay is you might blow the fuse to it with a short, then your engine will shut down.
http://h2o.localal.com