on the sierra the heater motor constantly runs. To stop it u need to push both levers (hot/cold and vent direciton) down. i was told this by ford them selves. Another thing to try is take the battery off over night and reconnect it in the morning and if it starts then u know it aint the battery, oh give it a good charge the day before and let it stand over night. The last thing to try is get a volt meter and check the outpu on the alternator by starting the car and registering the voltage between the battery terminals.
Good luck
James