Where is oil drain plug on 1999 Cadillac deville?

The oil drain plug on a 1999 Cadillac DeVille is located on the bottom of the oil pan, which is situated underneath the engine. To be precise, you'll find it on the driver's side (left side) of the engine.

However, you should consult your owner's manual for a diagram and precise location. The manual will have detailed illustrations which are far more helpful than a general description. Improperly locating and removing the drain plug could cause damage.