What was found
AIRCRAFT WAS GROUNDED: ON APROACH INTO DFW AFTER AUTOPILOT DISENGAGEMENT AT 1,000 AGL, RUDDER TRIM WENT TO FULL LEFT TRIM, KNOB WOULD NOT WORK. FLIGHT CREW RESET ON GROUND TO ZERO, BUT DUIRNG TAXI IN THE RUDDER TRIM WENT TO FULL LEFT AGAIN. MAINTENANCE REPLACED RUDDER TRIM CONTROL SWITCH 9CC PER AMM 22-62-11. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED.