GPS?

Put a PC in your car and call it a day.

You then will have GPS, DVD, endless MP3's, wireless internet, free XM(if you can find the wireing harness needed). I put together a system in a friends Eclipse for under $1100, and that was including the VGA LCD for the dash.

get a double din navigation unit off ebay. eclipse and pioneer make good ones. if you have a 2001+ year it will go right into the radio slot and look awesome with dvd, navi, radio, sat radio, and cd/mp3 playback.

Yeah they make some touch screen nav units that fit right in the double din enclosure. They look factory in most cases, do some more searching like others suggested, heres a pic of the screen they used in the SN65:

My Range Rover already had the Pioneer AVIC-N1 installed in it when I bought it. I love the Pioneer, and was looking at putting one in my Mustang as well. Pioneer does make a double DIN unit, the AVIC-D2 http://www.crutchfield.com/S-wKxSiFtGUXk/cgi-bin/ProdView.asp?g=182350&I=130AVICD2 . The one drawback between the N1 and the D2 is the N1 can play DVDs while the D2 (double DIN) can't.

I also saw on the Crutchfield.com site that Eclipse also has a Double DIN unit (the AVN5435 http://www.crutchfield.com/S-wKxSiFtGUXk/cgi-bin/ProdView.asp?g=182350&I=099AVN5435 ). I'm not familiar witht he Eclipse brand, but it does seem to have really good reviews.

Both of the units would look like a factory install in the Mustang.