several things here.
That big wire from the alt supplies all the power to your car, so you need more like a 50 amp fuse.
If it is one of the newer square backs then 80 amps will cover you.

Next, you need to make sure your vr is grounded. Then you need to measure sensing voltage at the vr.
That taps off the ignition switch, and that means it passes through the bulkhead connector.
That power passes from the ignition switch back up through the amp meter and then back out the bulkhead again.
At any point you could be suffering from a bad connection or part causing a voltage drop on the sensing voltage.
That is if a good ground doesn't fix your problem.