2000 Chevy S-10 dampness /wetness drivers side carpeting

I havent had this truck very long but i've noticed that since ive had my truck there has been a bad, musty smell in the cab. i took out the rubber floormat from the driver's side carpeting and noticed a VERY soaked carpetiunder the mat. i soaked up quite a bit as much as i could and today, it's soaked again. I rarely drive this truck (average 10 miles a week. it sits allot). i didnt notice any problems in any other area of the interior. ALSO, i took the truck to the service dealer to inquire about a "service engine soon lite" they said it was transmission related and suggested that i get the transmission flushed. i had them do the work for $200. a week later, the service engine soon lite is back on. any ideas?