I have just picked up an old Civic as I sell my 20VT and I have just connected the Civic's original radio - no idea what was in before.

Radio seems to be fine until I turn on the ignition/run the engine when the volume decreases. It's a twisty-knob volume, rather than digital and even when turned to max I am guessing it only throws out 50% volume. When I turn off the engine/ignition, the volume returns to the right level.

Any suggestions?

Problem solved!