Without taking the dash out-the whole heater would be a real pain. Try warming the car up fully and then-with the blower off-put you hand on the heater box. Also check that the hoses going thru the firewall are in fact warm or hot to the touch. Could be that the internal part of the valve has failed. Another thing-are you using the stock thermostat and the correct anit-freeze mixture? That is all I can think of.