Maintenance supervisor reported statements from a lead mechanic and a maintenance technician who inadvertently installed a C208B aileron trim cables in reverse which caused the opposite directional input to be reversed; which was discovered during flight.

Narrative

While performing a Phase 1 inspection on Aircraft X at ZZZ under Work Order #XXXXX; I observed that the aileron trim cables were loose and dragging along the bottom of the wing. Attempts to adjust the cables to achieve proper tension and clearance were unsuccessful. I determined that the cables were stretched beyond serviceable limits and directed that they be replaced.Mechanic Person A completed the cable replacement under Line #51. Following installation; the trim control exhibited binding. I suspected the actuator to be the cause and instructed Person A to replace it under Line #59. After the actuator was installed; I asked Person A to perform travel checks. He reported that the checks were complete and that they're good."As the RII (Required Inspection Item) mechanic; I inspected the safety wire and verified control knob movement. No binding was present at that time. I then signed off the RII for Lines 51 and 59; along with the remainder of the Phase 1 inspection package.On Day 0; at approximately XB:00 local time; Aircraft X departed ZZZ for the scheduled ZZZ1/ZZZ2 feeder flight. Shortly after takeoff; the aircraft returned to ZZZ. Upon taxi-in and shutdown; the pilot reported that the aileron trim was operating in reverse--right trim input resulted in left trim movement; and left trim input resulted in right trim movement. Maintenance subsequently confirmed that the aileron trim system was functioning opposite to normal operation.On Day 0; at XA:27 local time; I departed ZZZ on Flight ABCD en route to ZZZ1. During the initial climb; passing through approximately 500 feet AGL; the aircraft began an uncommanded roll to the right. I applied left control wheel input along with left aileron trim; however; the right roll became more pronounced while in IMC conditions.Maintaining runway heading; I then applied right aileron trim; which successfully leveled the wings. I immediately contacted Tower and advised that I was returning to the airport. I initiated a shallow left turn and reduced power to begin a descent; requiring minimal control wheel input.I broke out of the overcast at approximately 400 feet AGL into 1¼ miles visibility in mist. I advised Tower that I had the airport in sight. The controller responded; "I have you in sight--no need to reply." I landed without incident.Cause:The aileron trim system malfunction on Aircraft X was caused by a reversal of the aileron trim cables during maintenance performed as part of the Phase 1 inspection at ZZZ. The original trim cables were found to be stretched and improperly tensioned; prompting their replacement under Line #51. During reinstallation; the new cables were inadvertently routed in reverse. Although subsequent binding led to the replacement of the trim actuator (Line #59); post-maintenance checks did not detect the reversal.The reversed cable configuration resulted in opposite directional input from the cockpit trim control--right trim input produced left trim movement and vice versa. This condition was not identified during post-maintenance functional checks or RII sign-off. The issue was discovered in flight on Day 0; prompting a safe return to ZZZ. Maintenance confirmed the reversal upon inspection.The root cause of the incident was the incorrect routing of the aileron trim cables during installation; compounded by insufficient verification of correct directional control function during post-maintenance checks.Looking back; I did not verify the direction of trim movement during the RII. The cables were not clearly labeled inside the wing root; and the reversal was not caught during rigging or travel checks. I take full responsibility for the oversight and support the corrective actions being implemented to prevent this from happening again.Suggestions: Mandatory Direction Check During RII.Require all RII inspectors to verify correct directional movement of flight controls after any cable or actuator replacement. This should be a documented step; notjust a verbal confirmation.Improved Cable Labeling.Label aileron trim cables and rigging points inside the wing root with clear; durable identifiers to prevent reversal during installation.Two-Person Verification.Implement a structured two-person check for all flight control rigging tasks. One mechanic performs the work; and a second independently verifies correct routing and control direction.Stand Down Protocol.Support the rollout of the Maintenance Safety Stand Down feature in the company app. Mechanics should feel empowered to pause work and ask for help without hesitation when something doesn't feel right.Workload Management.Reassess workload distribution at ZZZ. Time pressure and limited staffing contributed to reduced verification discipline. Shifting some tasks to other bases could help reduce risk."