I bought a used 2002 Mazda MVP, when it rains, the engine misfires.What could be the possible reasons for this.

 

When its dark fill a spray bottle up with some water and a little bit of soap, shake well and then while your engine is running spray the water/soap over the spark plug wires and watch/listen for sparks, you will most likely find you need new wires. How To Check For Bad Spark Plug Wires - YouTube