It's almost assured to be the regulator at fault. If you set the fan speed to top speed you should hear considerable blower noise if the fan itself is working right even if little air comes out of the vents. If that's what you have then there may be a problem with the vent doors not opening, but if you crank the blower speed up to top and all you hear is what sounds like a "wisper" of a blower on low or medium then it's almost for sure a bad regulator which is very common on these. The fix is well documented here, you'll either need to retrofit the W140 regulator or replace the entire blower/mount/regulator with the new design from MB.

Outstanding thanks guys