Same problem with my 99 SL 500
Took it to an indie A/C shop.the blower will work some times and then it would not come on at all other times,the mech. dug it out of the car, this thing is buried in between the engine firewall and the back of the dash below the filter assy under the hood.I have all the parts that were replaced and it clearly shows that the double squirrel cage plastic blower housing MB pn.A 129 830 01 03 .cracked around the (plastic) retaining clips allowing the squirrel cage to rub against the housing,evidently slowing or stopping the blower from turning depending on the selected fan speed.never heard any scrapping noise that will point this out so I was lost trying to diagnose it.This is not an electrical motor is an electromagnetic contraption that looks like the cooling fins on a motorcycle engine, this is activated by a control module another cooling fin looking contraption,BEHR 129 821 33 51,that burned up trying to keep the blower turning, This was $900.00 parts and labor with original equipment replacement and after examining this cheaply made parts that I will expect to find in a Hunday it a big possibility that you are having the same problems.GOOD LUCK