What could be wrong??? Possibilities please

[Fantom]

Ok guys, having some trouble with my stereo. Its a 2003 172.
Started off by being a bit intermittant. The stereo would just not work, but by this I mean no sound comes out but everything else is as normal. So you turn it on and off and eventually it works again. Kept doing that for a while. Eventually it stayed off for weeks.
So I bought a new head unit from ebay. And still no sound!!!
So there must be a dodgy connection.........cos I doubt all 6 speakers blew up.
So what other connection is there behind the dash? Is there anything other than the plug that goes into the head unit?
Someone has definitely monkeyed about with the standard wiring cos the plug has been cut off and then put back. All been soldered though and looks ok.
Is there any thing else I need to look at?
Gonna have to take it all apart me thinks

 

[cliocc]

Not sure about the standard head units but the likes of Alpine and Pioneer head units have a cut-out which turns off the internal amp (and so the sound) when there is a short circuit on one of the speaker wires. The short could be to another speaker wire or to earth and would have the same result. Ive also seen it happen with a burnt speakers. Use a multi meter to check for resistance between speaker terminals on the speaker plug when its dissconnected from the head unit. You should get around 4 ohm's between each pair of terminals and nothing from any terminal to earth.