OK... tired of chasing this.... 2004 Silverado...5.3. HVAC fan not working as the control unit stays black, but the temp ***** on the left and right light up. Control unit was recently replaced and worked fine, but did not totally fix the problem. Now it's acting up. When stopped, there is no air coming out of the vents. When rolling, the air is coming out cold as if the A/C is blowing.on low fan. It is not. It's 85 degrees outside. This seems to be outside air that is being cooled since there is no fan blowing. When it was working correctly, I could turn the thermostat ***** to 90 and the air would blow hot even with the unit turned off. I could lower the temperature with the ***** and whatever the temp was reading is what the air temp would be. Again, this is with the unit turned off. Here's the real kicker with this. If I leave the vehicle turned off for more than two days, the battery is dead and the vehicle won't start nor will the dome light come on when the door is opened. Something has to be running non-stop... but what? The battery is less than six months old so that's not the problem. It's got to be electrical, but anyone I speak with says chasing electrical gremlins is not fun. Vehicle has 412,000 miles on it, but almost everything is still original. Any thoughts on my electrical problem?