Mercedes 280se Starter Question

Thank you for reading. Question is on a 1972 280se. Car was running fine. I stopped at a shop went in, came out 15mins later car would not start.
Issue is that everything electrical seems to work fine, except for the radio which worked before. I turn the key and I get a single click under the hood and hear the fuel pump whirring.
Any ideas? The other side of this is this: I notice that there are 3 wires on the starter, one is disconnected (corroded through). If I reconnect this same wire to its original point, I still get nothing. Thought I had figured it out when I found this wire, but guess not. Can someone tell me if there are 3 wires on the W108 that need to be connected or is it just the two and if so what are they for. Thanks again!