If you can get to where the 12v comes in, and disconnect that, it will not matter if the new 5v regulated feeds back to that contact. The 5v needs to feed to the gauge movement heating winding in the fuel gauge, and then on to the 2 other gauges. What you can't do is feed the new 5v into the old 12v terminal.
If you have the o'scope, I would do that first to make sure the old IVR is the issue. BTW, the pulse duty cycle may be fairly ragged and somewhat erratic but still be OK so you have to look at it for a while.
