Dont waste your time with a cheap low level converter, do it right and get a JL Audio Clean Sweep, Chances are that Stock audio system is tied into other componates in the car. A cheap low level converter will sound like crap.
I had a cheap one in my audi on a bose system and it sounded horrible. Spent the money on the mid grade JL Clean sweep and it was a difference of night and day with the same equipment.
Oh and take it to a proffesional, that way if they mess something up it is covered, if a friend or you or some joe smuck tries to install it and screws something up then you are screwed and more issues/money.