the other option that comes to mind is an opto isolator in parallel with the injector.
This one would need a little protection on the input (2v zener and at 250 ohm resistor at least),
NTE3045
w/250 ohms, it would only be at .008 amps if the injector holds at 2v. The test conditions list .01 amps so with the built in darlington it might be useable. The darlington collector emitter can handle 80 volts, so it *could* be doable with just 3 components, maybe, and a plug in. Like $3 in parts.