Cathaldub is right. Its the glue from the digitizer melting. Everyone that has posted here has the same issue. Basically your car heated up and the di-electric glue pioneer used is NOT designed for a car.
DO NOT USE A BLOW DRYER. Blow drying it or heating it up will just make it function worse. There is basically a plastic grid of wires that sends a signal to the radio where you touch the screen on that grid. The glue both allows a connection and holds the plastic and wires in place.
The options are:
1) never buy a pioneer radio again, they're not going to start putting the correct glue
