A/C & heater problem

Were I work our trucks are our mobile offices. A/C does not works between dealershp & another repair shop replaced heater core(twice), flushed the cooling system 2+ times, new radiator–A/C will work for a while then quit same with heat in cold cold weather–Boss won’t spend any more $$$ on it (3K so far)HELP–great truck unless it is cold or hot out!! Not that may parts involved but I feel they are neglecting to check something but what??