On February 25, 1994, about 2223 central standard time, a British Aerospace ATP airplane, N854AW, operating as scheduled passenger flight 5033 under 14 CFR Part 121 by United Express, Inc., experienced in-flight smoke in the cockpit as it approached Mosinee, Wisconsin. The flight landed successfully at Central Wisconsin Airport (Mosinee), and the fourteen passengers were evacuated by use of the left side cabin door slides. Two of the passengers reported minor scrapes and bruises. None of the two flight crew members or two cabin attendants were injured. The flight departed Chicago's O'Hare International Airport at 2045 and operated in visual meteorological conditions. An instrument flight plan was filed for the flight. The flight crew reported the airplane was descending through 12,000' to 8,000' msl when they smelled smoke in the cockpit. Shortly thereafter, the cabin attendants informed the flight crew that they also smelled smoke. The flight crew initiated and completed the emergency check list for fuselage smoke, and then discovered a red-hot, push button selector indicator (PBSI) switch located on the copilot's right side panel. The copilot discharged a fire extinguisher on the switch, and the smoke stopped. United Express maintenance personnel removed the flight idle baulk test PBSI switch and found that adjacent wiring was heat damaged, and that the switch body was burned and partially melted. The switch was sent to the NTSB for examination. Conversations with representatives from the manufacturer disclosed that failures of like PBSI switches in the cockpit had been reported, and that the switch failures had been attributed to water entering the switches from the side cockpit sliding windows which were left open in inclement weather during ground operations. The manufacturer has issued Service Bulletins SB 56- 3, dated October 30, 1992, and SB 24-53, dated March 22, 1993, which call for the operator to modify the side trim panel consoles to divert water from the switches, and for an application of a silicone sealing gel to the switches. The incident airplane had not complied with the above Service Bulletins at the time of the incident.