DOT is proposing a $325,000 civil penalty against SWA for allegedly operating an aircraft that had been improperly modified, violating Federal Aviation Regulations.On Aug. 29, 2011, maintenance personnel improperly installed a switch that enables flight crews to test the windshield heating system on a Boeing 717 that AirTran Airways Inc. was operating. Southwest is in the process of merging with AirTran.Proper installation of the switch would have allowed personnel to isolate the windshield anti-ice system that was causing a warning that the windshield heater was failing. Instead, the center and left windshield warning systems were reversed. The right windshield warning system continued to operate properly. The aircraft was operated on 1,140 passenger flights before the problem was corrected.