Your best bet would be to pick up a new headunit with an aux-input. It'll sound better. It'll most likely be cheaper and it'll be less work. Adding an aux-input to a factory headunit usually requires cracking the unit open and soldering to tiny leads that are often unmarked. One false move and you'll be buying a new headunit anyway when you break the factory unit. Honestly, unless you're a tinkerer and you like to take things apart, it's more trouble than it's worth.

Save yourself the trouble and treat yourself to a new headunit. A unit with a line in can usually be found for $99 or less these days. Or spend a little bit more money and find one with a USB port for even better quality.