My wife just got one installed in her Jeep Liberty, it sounds great. A little pricey but the add on display is super cool. I haven't been able to find out if it can work with Mustangs yet. The iCruze site doesn't have it as available for the Mustang. If anyone knows or has it installed in their Mustang let us know.
If your gonna have a aftermarket HU, don't even bother with the monster icruze. Look for an adapter off crutchfield to input your mp3 player.
For example, I have a Alpine HU. I purchased a cable converter to convert the cd changer signal into a RCA signal. And then from there I went with a cable converter from RCA to earphone port and plugged that into my iPod. Now my Ipod plays thru my AUX input on my Alpine HU and I am able to tweak the music with all the alpine settings. Total cost was about $30.
Do some more research on it to see what works for you.