Further information: I think I may have found the issue - the red permanent power to the amp (via the antenna?) is only giving me 2V...

Hi folks,
A problem of two halves, this one:
My stereo stopped working, shortly after a battery fault. Garage said it needed a new HU, so I bought one (not Vovlo specific - a Sony model with BT etc). This doesn’t work either. Have also tried replacing the amplifier in the boot, but still no joy ...