Factory Amp vs Aftermarket Deck Amp w/ Bluetooth

[Keddie]

Does anyone know what the Factory deck specs are for RMS power and what is the impedence of the speakers? I'm determining whether to have a stereo shop integrate the factory AMP or to use the deck power. I an external amp is normally preferred but like the new Alpine decks use amps put out 18W RMS at 4 Ohm's.
http://www.alpine-usa.com/en/products/product.php?model=CDA-9857


Also has anyone had aftermarket bluetooth integration into their cars? I'm seriously considering this because that would be sweet! Here's the link
http://www.alpine-usa.com/en/products/product.php?model=KCA-100BT

Thanks

Keddie