Right - Think im there with this although i have been lucky in my case.

If your's is the same as mine then you have no seperate amp, the radio module takes care of all audio. I spoke to a automotive electrical enginner who described the problem to me perfectly, he also said that if it is the radio module that controls audio, then there will be 6 pairs of twisted wires where the speakers connect into it. He said the only way to know for sure if the module is damaged is to get the car plugged in to the diagnostics to find out. I found a specialist in Milton Keynes who proptly told me my radio module was in fact knackered. This is where i've been lucky - Since they told me it was dead, it was worked flawlessly - no idea why, perhaps they reset it
Fingers crossed it stays that way.
I would suggest you get it plugged in, if nothing else than to be sure it is dead. The garage i found only charged a 30min fee of £26.50
If that doen't work for you, I found a company that can repair these units.
They can only repair the BM24 units (Has a heat sink 3/4 the way along it's length)(which mine is and im guessing yours will be) and charge £200 but they told me they have a 100% success rate
Car Audio Repair Service 01273 734085
www.caraudiorepairs.co.uk

Hope that helps

Rich