Ford fuel injectors are controlled by the Computer grounding them, so on a V6 there will be 6 ground wires from the computer, one to each injector.
When key is turned on all the injectors get 12volts via 1 wire, it is usually a Red wire and it is "daisy chained" from one injector to the next.
This is the wire you would want to put a relay on.
This would cut the 12v when running on LP.
Battery-------EEC Relay-------Your relay---------injector 1-------injector 2-----ect.........
EEC Relay powers computer(PCM), fuel pump relay and several sensors when key is turned on, so you can't use it, you will need to trace the main power wire from the injector side and insert relay there