Intermittent ventalation system fan control problem

Hi, my 04 Buick Le Sabre has an intermittent problem with the fan and directional control of the air coming out of the dash. The air output will randomly switch itself from air coming out of the dash, to the floor, to the front outlets without any warning and without me pressing any of the selection switches. I was told that it might be a vacuum problem such as a cracked, broken or rodent chewed hose causing the air flow to the baffles to fluctuate? Any thoughts? If it is vacuum related, I need help identifying the correct vacuum hoses that run to the in-dash air system. I am not having any luck finding a schematic on-line. Also. Any thoughts that it may be an electrical issue? Last summer I was experiencing the same problem with this car, as I have just described. Then about a month after the problem started, I replaced a burnt out right tail light bulb and suddenly, the air flow system started to work perfectly again! I have checked all of the perimeter bulbs on the car and do not find any that need replacing? I think my car is possessed! Maybe I should call a priest! Any thoughts and suggestions you have will be greatly appreciated> Thanks. John