Do you have the premium sound? If so, your little 5" Ö. thing (sorry, I find that 'sub' really sad :rolleyes
will have power problems. If you need another ground for some odd reason, all you need to do is run a wire from the ground on the wiring harness to the frame of your car.
Personally, I think you should do the install yourself. It's amazingly easy - the hardest part is unburying your radio, which varies in difficulty by car. Once you get your old radio out, you simply plug the wiring harness (anyone that does installs will sell you a harness) into your stock harness and then plug your stereo in.
You will have to solder something like 12 sets of wires between the two harnesses (the two new ones Ė one to go to your carís stock wiring and one to the new head unit), but thatís color-coded. Black goes to black, etc.