Starter Troubles still

I just cannot find out what is causing my solenoids to go bad I have replaced the starter three times already. One was a cheepe and the other two were Bosch rebuilds. I have run a wire from the starter solenoid to a switch to the battery and I am still having problems. I have to hit the solenoid to get it to work. As soon as I put the latest one in I had to hit it just to get it to work. I have a new battery new battery cables and my engine is grounded. My alternator is putting out 13.7V and by battery with the engine not running is putting out 12.7V. Can any one help? By the way the car is a MB 240D 1983.