My '73 450 SL has been in the shop for 4 weeks and they are scratching their heads as to why it won't start. At first it would happen after the car had run for a long time and so we assumed the alternator wasn't recharging and so we replaced it. But the problem persisted. Next we replaced the starter. And now, it is completely dead and won't start at all. The battery is a year old.

Any ideas?

Here is a video of what happens when I try to start it: https://www.dropbox.com/s/loivqp9sjh..._1922.MOV?dl=0

 

The sound the first time reminded me off corroded connection(s). Make sure all wiring and battery connections are clean.

Second time with no sound makes me think of failed ignition switch. What are the dash lights doing when first turned to on?