Volvo 480 Club Europe

I'm interested in popping some extra speakers into a home-made rear shelf but want to ask about the electrics. Give the 480's delicate circuitry I want to be minimally invasive, I'd really prefer not to run an extra cable to the battery to power an amp etc, is there anything a bit less 'max- power' that could just split & amplify the output from the head-unit- or are there any head units that can power 6 speakers rather than just 4?

Thanks!