the 99 uses a one way clutch pulley on the alternator so its a lot less stress on the belt tensioner system.

The arm can also wear out if its really old and the tensioner shock has been loose all along. I never could understand why MB thought that a metal on metal tensioner with no lube is a great idea but thats what is on the engine.

Its best to check the arm for wear, to do this - release the tensioner and try wobble the arm with some leverage, if you can hear it click - its gone - get a new one. Really bad ones can make the belt slide forward on the pulley.

speaking of costs of ownership

All cars cost money to run and if you are someone that cannot account for downtime or other costs - then such cars are not good, best get a lease which can also provide an instant loaner everytime you put it in the shop.

I have a regime to test the car every month, e.g. When I changed the brake pads I repacked one front wheel bearing as the grease looked compromised. Just a while at it job - similar cases exist nearly everywhere. And a good mechanic can also check those for you.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)