Sounds like it might be some form of a factory amp for a car that uses negative switching to switch the device on.
To get it to work properly, you may need to get a relay to act as a switch to supply ground to the normal ground wire when the radio is switched on. I'll make a diagram later and post it here so it will make more sense. Hopefully.
When you say theres 8 wires which is all good, does it have these wires that hang out of the amp?
And does it have RCA plugs on it? Or do you just wire the speaker wires from the deck straight into the amp?