[Richard L]

The red wire of the grey harness powers one side of the relay coil, the other side of the coil is grounded. The relay is not a latching type.

It is very unusual not to have 12V present at the injector(+) at the pre-crank position.
Some cars switchje doff the (+) rail of the fuel injector after 10 seconds or so if no cranking has taken place during this period.

You might have one of those cars.

I am not comfortable for you to power-up the relay other than the fuel injector positive. It can cause false reading on the IDC signal.