Heater core

I have a 1990 chevy lumina and have no heat i replaced the thermostat and flushed the coolant and still nothing im guessing its the heater core and was wondering how hard it is on this car does the dash have to be removed?

with the car running at normal temp grab each heater hose they should be the same temp(hot) if not then you need to ck heater controls if every thing ck out ok then you have a clogged h/core