What grassroots said, it should say something on the speaker near the window corner. I had a 95 mustang with mach stereo that I added a head unit and subs to. The part that you need can be found at wal-mart for under $20.
http://www.walmart.com/catalog/produ...uct_id=1863065
I am 99% sure this is what you need. I assume that this will work with 99+models. I know that it will work as long as you have the right adapter. It allows you to connect each individual speaker wire (+&- for each speaker) into it and adjust the signal strength/level, or whatever you want to call, it to your liking as well as adding rca outs.
http://www.walmart.com/catalog/produ...33274%3A164122
This is only up to 97 so I am not sure if the connections are still the same on later models. I was lucky and had a 95 so everything worked fine. Unfortunatley, I sold all my stereo stuff with the car so I forgot exactly what it was. You can always try it out and if I'm wrong, return it to wally-world.
Unless you want to splice up your existing wire harness, I would go this route. It's safer IMO cuz if it doesn't work, which it should/will, it was a free trial and nothing got messed up. Trust me, I spent a few days wondering why my more powerful HU wasn't making the car go BOOM BOOM until I got the "line level converter". Then my license plate rattled just fine
