[timpy]

Could be a bad connection in the amp itself, either at the connector, the other side of the connection inside the amp, or at the input circuit stage with a failing component or bad solder joint. This could be on the PCB itself and therefore is an amp fault.

However the electronics are likely to be far more reliable than the connections, there doesn't seem to be a trend on here of faulty amps for their solder joints; and don't lose sight of the fact that it changed behaviour when you moved the lead. If this was at the amp end, I'd concentrate efforts there, wiggling etc. with the new or old lead. It sounds as though you've changed the entirety of the signal lead, does it run the length of the car on the C70?

Is it better or worse with the car warm inside, or having cooled over night?

It could be at the output of the head unit also of course. Audio signals are small before they're amplified to speaker level by the amp, and far more prone to being lost through poor connections.

For your amp, if you have an mp3 player, have you tried injecting a signal into it from the headphone output. There may be a little jiggery pokery connecting it up, keep the volume low to start with (!) but it should give an idea of amp health. I wouldn't try it whilst driving obviously.......