PA-34 pilot reported the pitch trim failure during cruise. The pilot disconnected the autopilot and with the help of a pilot/autopilot onboard were able to continue to destination.

Narrative

After climbout from ZZZ; the autopilot struggled to hold altitude and got to about 150 feet above assigned altitude before I intervened and disconnected the autopilot. When I disconnected the autopilot; the airplane pitched up and I could immediately tell that the airplane was out of trim. I adjusted the manual trim wheel nose down and quickly realized that the cable was disconnected. I requested a block altitude from ATC and informed them of a 'trim issue.' Afterwards; the 20-30 lbs of forward pressure required on the yoke was reduced by reducing power and having the front seat passenger assist with pushing forward. A pilot/autopilot was in the back of the plane and was able to access the pitch trim control cables. He was able to trim the airplane out for a slightly slower cruise airspeed; and the autopilot was once again able to be engaged. From there; we called nearby airports and found a mechanic who would be able to look at the issue at our destination. The rest of the flight produced no further complications; and there was no problem getting the airplane on the ground with the assistance of the mechanic in the back.