Why would my Suburban crank but not start and backfire and no codes are showing up?

My 1999 suburban started to run rough about a month ago and I put new spark plugs, wires, rotor and cap on. About a week ago it would crank but not start and it backfired I replaced the ignition moduler and it started right up it was running rough so I changed two spark plugs that had alot of gas on them and then is started and ran great after it warmed up. Yesterday it rained and it would crank but not start then it backfired again. This problem seem to happen when it is raining or wet outside. There is also a leak in the windshield and water is getting in on the passenger side were the glove box is located so I am not sure if the computer itself has been runied?