What was found
(CAN) AFTER DEPARTURE, STAB TRIM AND MACH TRIM MESSAGE APPEARED, STABILIZER WAS STUCK AT 8 UNITS. UNABLE TO TRIM, AIRCRAFT RETURNED TO TORONTO. FOUND FIRST OFFICER DISCONNECT SWITCH AT FAULT. CHANGED FIRST OFFICER STAB DISCONNECT SWITCH, FUNCTIONAL CHECK OF HORIZONTAL STABILIZER SYSTEM CARRIED OUT, NORMAL.