Yes you can hook it up to your existing deck, but it depends on the connections in your existing deck.

"How does the line input work?"

Im not sure how to answer this, but you plug in 2 rca cables, and choose aux to hear whats coming over those cables.

"Can I use the existing deck as a slave to riocar and be able to use both decks? "

I don't know if you will be able to do this or not, since I don't know what stereo you have, I dont know if you can fit 2 din sized units in your dash, and I dont know what you mean by a slave.

Sorry im not too helpful, but all I can too with the info I have is speak in generalities.