What was found
AIRCRAFT WAS GROUNDED, ACFT HAD UNCOMMANDED RUDDER TRIM INPUT. HEADING CHANGE OF 2, TRIM CHANGE FROM NORMAL L 0.3 TO L 1.4 UNITS. WITH BOTH AUTOPILOTS YAW INPUT, WORSE ON AUTOPITOT 2. BETWEEN UNCOMMANDED RUDDER TRIM, HAD STATUS MESSAGE CAT3 SINGLE ONLY INOP. SYS CAT 3 DUAL. NO OTHER INDS. CHANGED AUTOPILOTS. WENT AWAY, SECOND UNCOMMANDED RUDDER TRIM NIPUT. MX REPLACED RUDDER TRIM 1 RELAY 7CC1 IAW ESPM 20-46-50. ENGINEERING FOUND THE RUDDER TRIM ACT CURRENTLY INSTALLED ON ACFT WAS REMOVED FROM ANOTHER AIRCRAFT FOR SAME ISSUE & WAS SENT FOR REPAIR PRIOR TO BEING INSTALLED ON THIS AIRCRAFT & ENGINEEERING RECOMMENDED IT BE REPLACED. RUDDER ACTUATOR REPLACED IAW AMM 27-22-51. OPS CHECKED GOOD.