Ford Fiesta 1998 Zetec - Radio hates sunlight! (gremlin?)?

Thermal problem in the internal circuitry. I bet if it's working and you crank up the heater or warm it with a hair dryer it will quit. This kind of stuff happens once in a while. There is a faulty connection either inside an integrated circuit, on a circuit board, or with the solder connections between components and the board. When the radio is below some temperature threshold the connection makes and the radio works. When the temperature is over some level the connection is bad and the radio fails to operate. This is caused by thermal expansion and contraction of internal parts. This kind of stuff usually costs too much labor time to troubleshoot to the component level so the whole radio is replaced, particularly on something 13 years old.