Aftermarket subwoofer

Hey all, I have bought a new 2005 Yukon Denali with the navigation and I want to add a powered subwoofer.

I have a friend with a new Acura with navigation and all he had to do was get a low level line converter for the sound (along with the normal power, ground and remote wires).

A local stereo shop, the only one with JL Audio, said I would have to spend $300 for a crossover and wanted to sell me a $1,600 subwoofer and amp, that is only one JL 12 W3V2 and a JL 250/1 mono amp.

That is outrageous and I want to do it myself with the same components for about $700.

So the question is, do all I have to do is get a low level line converter or do I need something special to get the sound from the deck.

Thanks in advance for any help I receive.