Audio guru help needed

Story goes, my last Octavia was fitted with an aftermarket headunit which had an aux pre-out for which to attatch an amplifier to. This was all fine and dandy, but in my new Octavia, I want to keep the Symphony headunit as it's got the multi changer etc. I would like to have the amp in the boot again, but the problem is the Symphony doesn't have any pre-outs.

Is there a way round this? For instance attatching the audio feed for the amp, to one of the rear speakers? Or would this over strain the current system or produce poor sound etc?

I know the most snesible option is to fit the aftermarket headunit, but then I would lose use of the multichanger, plus it looks garish and completely non-oem sat in the dash.

Many thanks.