copied this from a chevy site
Posted: Tue Apr 12, 16 11:24 pm
Pretty funny 
".....I had a similar problem with my girlfriends Chevy Nova a few years back. I replaced the heater core in the dead of winter and forgot to put the screens back over the cowl vents on the firewall. That passage covered by those screens leads down into the heater box. When the hot weather came around, the inside of the car stunk really bad. It was far worse when the fan or AC was on. Well anyway, I took the dashboard apart trying to figure out why my girlfriends box smelled so bad and I discovered a dead bird inside."
".....I had a similar problem with my girlfriends Chevy Nova a few years back. I replaced the heater core in the dead of winter and forgot to put the screens back over the cowl vents on the firewall. That passage covered by those screens leads down into the heater box. When the hot weather came around, the inside of the car stunk really bad. It was far worse when the fan or AC was on. Well anyway, I took the dashboard apart trying to figure out why my girlfriends box smelled so bad and I discovered a dead bird inside."
