I just put in a double din dvd and the sound sucks with a harness, do i need to rewire the entire car or is there another way with the mach 460?

 

Assuming your DVD player has front rear and sub rca outputs you can use this and it works great.

This is what I use in my Mustang. Im pretty sure the plug on the Mach systems never changed but don't quote me on that.
If it did change print this off take it into the place you intend to purchase it from and tell them you need the equivilent for an 03.