i just recently bought a 91 4 runner 6cyl. when the heater is on it blows very little warm air. I flushed the lines and that didnt work. Im gonna change the thermostat next if that doesnt work does that mean it is the heater core or could it be something else?

 

id try blasting your heater matrix out with a presssure washer to clear out any crap that might of accumlated in the matrix, blow through both pipes till the water is clear then refill your coolant and bleed it through also do the same with your rad incase that is blocked with some debris, if its still cold id go change thermostat.