Easier to do this job on a W126 chassis. Air conditioner hoses are in the vicinity on a 300D. Expect it to take 3 hours, but may be less if you can get the lower tube out quickly. I had to cut the grommet as best I could and pull pieces out with needle nose to get enough slack for me to pull it out of there. May also help to warm up the grommet in hot water before assembling to make it more pliable.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)