Since, I don't plan to use the CarPC I am building only for Mp3 playback I decided to substitute the mouse with a touch pad (I am already used to them from the laptops) . I still needed a device to be able to control easily Winamp - something like Irman, a small numerical keyboard or something custom made. Looking for various utilities I found a software and it's Winamp plug-in which I thing you might find an interesting alternative. It is called Sensiva (www.sensiva.com) and it lets you control other software drawing simple symbols on the touch pad, tablet or mouse. It is like the "Graffity" input method the Palms have. If somebody doesn't like the existing symbols (or he can't remember them easily) he can always customize the program (for example, making the "Play" function like that: > or the "next" like that: / ,etc...)
I picked up an awesome little thing from ubid.com that is perfect for car PC's. It is a touchpad, multimedia hotkey, and numerical pad all in one. It comes with some decent software to control it and is just a great little device to control anything in windows. Problem is that it bigger than the usual touchpad and off brand, but it's worth it.
I don't suppose you could tell us the name of this touchpad device?