What could be wrong if the engine of your 1999 Beetle putters and the engine light comes on when you start to drive it after it has rained?

You need to replace your spark plug wires!! You can check this by spraying water on your spark plug wires when it is dark. Then start your engine and you should see the wires arc and your engine will be running rough just as it does when it rains or after a rain. Good Luck