To expand upon Joshhol's question, MB recommends that you use a non hardening sealant when installing their window channels. MB sells their own sealant, but most here agree it is similar to this product from 3M:
3M Auto Bedding and Glazing Compound 1/10 gal cartridge 08509

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Joshhol: My tube of 3M 08509 is arriving today.

Originally I had read that you don't need to use any sealant. But then anytime anyone would have a leak, the answer always seemed to be to shoot some sealant in there. Then when I read that the factory shop manual for the W123 recommended sealant after install, I decided to do the sealant. I have a W115 from the year before the W123 was introduced, 1976, so I figure it's pretty applicable. I'd rather have a better seal due to sealant and deal with the mess it makes later down the road. Better than gradually getting water in the car and not catching it.

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76' 240D