Why do I not have heat in my truck. I have checked everything I can think of.

WELL IF YOU DONT HAVE HEAT IT COULD EITHER BE THE VALVE THAT CONTROLS THE HOT WATER TO THE HEATER CORE IS NOT WORKING, REPLACE IT, MAYBE A VACUME LINE TO IT IS NOT ON OR CRACKED, THE CONTROLS THAT OPERATE THE VACUME TO THE VALVE IS NOT WORKING PROPERLY, IF ITS CABLE CONTROLED THEN THE CABLE BROKE OR THE VALVE IS BAD. OR IF ANYONE HAS EVER PUT ANYTHING IN THE RDIATOR TO STOP A LEAK THEN IT PLUGED UP YOUR HEATOR CORE, YOULL HAVE TO UN HOOK YOUR HOSES AND PUSH AIR THROUGH THE CORE BACK WARDS AND HOPE IT UNPLUGS IT IF NOT REPLACE THE CORE. OR A FLAP IN THE DUCT WORK IS NOT OPENING.