there are plenty of people who use carputers without touchscreens. you can mount tactile buttons for your primary features like song changing, volume, gps, ect.. with a serial port connector and girder.

my touchscreen died and mp3car refuses to help without me spending hundreds of dollars for their 'repair' services, when i can just get a whole new lilliput for $260. the screen still works, but when it won't calibrate and when i touch it, the curser goes to the corner. nevertheless, i am now using a $12 touchpad from ebay.... not the safest thing when driving, but i dont interact with my carpc a lot while im driving anyway. im considering making tacticle buttons and soldering a serial connector to do my common features.