Amp remote wire goes into head-unit. It is possible that there is something wrong with it
But measuring voltage directly at head-unit may be difficult, as amp wire is in the same connector as power and front speaker wires. But connector and pin is quite big. I think you should be able to get voltmeter probe into it. That is pin 9 on connector. Here is pinout when looking from front of connector (wires away from you):
1 2 - - 3 4
5 6 7 8 9 10