Audio Help

[cbadbox]

Excuse my lack of audio knowledge,

I have read several of the DIY audio upgrade threads. Not sure what my 99 boxster originally had. Currently it has an Alpine CDA 9820XM unit installed. In the trunk there is a stock amplifier all I can read without taking it out is HAES 4X40 Verstarker. No rear shelf speakers.

I have purchased the Alpine SPS-410 for the dash and read the replacement thread. So I believe I am good to go on that. My question is whats the next best move? Replace door speakers or do the rear shelf speakers. If I understand it correctly the factory amp powers only the door speakers?

Based on internet research output on the Alpine unit is 18WRMSX4. I just want to make the most of the existing setup. I am not looking to redo the whole system.

Thanks