Late to this party but I will add my two cents for future reference.

I replaced my switch after exhibiting the typical intermittent non start. As far as I know it was the original 20 year old switch. That immediately solved the problems. The original switch on the exterior looked fine with no visible damage or wear.

A little over a year later the new Mercedes sourced switch went bad. Only 15 months later (unfortunately they are only warranted for 12 months). I needed the car and didn't have time to mess around so I just bought a new one and kept the old to look into at a later time.

Well today I opened up the switch and what I found explained everything.

Basically the device is very simple as illustrated in the photos above. The main problem that occurs is oxidation or dirt and grease build up at the contacts.

This is all exacerbated if the unit is defective from the factory in that moisture gets in and corrodes or gunks up the contacts. Which is precisely what had happened with my new unit. Had I known this I could have saved myself some money. Now I know.

In the future should I have another starting problem I will simply open up the unit, clean the contacts and be able to reuse it without having to spend more money on replacing a device that should offer years of service with little or no issues.