Why is my car dashboard overheating and only blows hot air after long journeys? (Ford Fiesta Ghia 1999)?

I made a 5 hour journey and the entire dash started to get really hot but the engine itself didn't overheat. It then also only blew hot air for a while which continued to blow regardless of whether the heating was turned on or off. It did then seem to sort itself out but the problem does come back occasionally. I recently had someone fiddling around fitting a black box to my car:/ so wondered if he may have caused a mechanical or electrical problem? The strange thing is that it does produce cool air during short journeys so means the heating can surely not be stuck on hot? Which seems to be a repeating problem with this model of car.
Any help with ways I can diagnose and fix this are greatly appreciated!

Sounds like the heater control valve, which is a notorious Fiesta problem. It's on the bulkhead at the back of the engine bay and following the heater hoses will lead you to it. Make sure the engine (and coolant) is cool before you start disconnecting it. He may have dislodged the electrical connection or it may just be coincidence that it's decided to fail now.