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My first long post was mostly speculation, sorry if that confused people. When I was messing around with the shunt yesterday, I was measuring the voltage across the shunt, yes.
When I tried using the rectifier and grounding the negative side, I could no longer get the nice waveform of the current.
So, as dcb said, isolation is the issue. Neither side can be grounded, but the sides have to be compared to one another.
I just went and looked at the post that dcb linked to, and it gives me an idea. It appears as though that schematic you drew uses the ECU side of the injector as a switched ground. It seems as though that would make sense, because at least on my truck, the injector is put onto 12v and the other side goes to the ECU, which grounds the injector to activate it.
So, it may be possible to just put a very low current draw device on the lead to the ECU, so that when the ECU grounds the injector, it activates said low current draw device. I suppose a diode would be in good order so that the 12v (when the injector is not grounded) doesn't mess anything up.
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