its in a 92 mustang and it overheats ina bout 15-20 minutes, could it be the waterpump?

the heater core isnt working also, could that be the problem?

Swap the waterpump and thermostat and it should fix you right up. That is unless your radiator is clogged. My old 89 5.0 did the same thing when I first bought it though and a waterpump and a new t-stat fixed it right up.