so if the gas and temp gauges don't work...

Yes but not necessarily the IVR. Could be a bad connection (power) feeding the cluster, or a poor connection right at the IVR. Obviously the quick way is to plug in a new one.

What rocky was talking about, is the earlier bodies (before 66?) had the IVR built into the fuel gauge. Also, all the Rally cluster ones do as well
yep sorry should have specified I meant my '72 standard gauge assembly. Is there a quick check for the IVR? (I take it one side would have 12v and the other would have 5)
thanks guys.