1989 Merc Starter Problems

I recently experienced a problem with the starter continuing to run after the engine started. When the ignition switch is turned off the starter continues to run. I had to disconnect the starter cable from the solenoid to get the starter to stop. I have replaced the solenoid and the starter motor. This seemed to solve the problem, but after a few starts, the problem returned. What else can be tested/checked/replaced. Please help. I'm lost.......

Replace the ignition switch.