Weres the gas filter on 1999 cadillac deville?

The 1999 Cadillac DeVille's cabin air filter is located behind the glove box. You'll need to remove or at least partially remove the glove box to access it. There's no engine air filter easily visible; it's integrated into the air intake system and requires more significant disassembly to reach. You should consult a repair manual specific to your 1999 Cadillac DeVille for detailed instructions.