May be of use to someone else so I thought I'd take pics when mine started playing up.
It was just 4 x M6 nuts (10mm socket) to get the unit out, two are obvious when you pull the vanity pannel off, the other two are holding the bracket and are behind the wire harness & squirter hose that are held in place with cable ties.

You'll need a T30 & T10 torx bit.

Whip out the 4" cones and remove the speaker wires.


The box pry's open quite easily and with 4 more T10 torx, the amp board comes out.

For me, this was the culprit.

The soldered terminals had broken loose as the plug is not well supported.

I re-soldered the 6 terminals and put it back together and all is well

 

What was the problem? No sound from the sub unit? Thats a nice repair, trick. The bose systems always seem to have an easy fix. On the B5 A4's the rear deck speakers blow up, or at least what sounds like a blown speaker. When you take off the rear deck you find that the cone on the speaker has come loose, fixing that just requires some super glue LOL. $250 per speaker my ***.

 

Yeah, it was working intermittently and would go very crackly before going out, this was pointing me to the speaker connections more than the amp circuit.