Not sure which part your talking about.Soooo

Corrugated tubes from the heater box to the vent assembly. Then are just push on. There are small nubs that hold the tube in place.

Rubber boot from heater box to center vents. This requires a little dexterity. I use a Small screw driver to move the edge onto the heater box as well as the vent assemblies. Place as much on the box/vent as possible holding it with your fingers as you roll it on. Some silicon spray helps it slide on.

The actual vent assembly as mounted on the dash. IF you have broken any of the small opening tabs on the black vent assembly. It will not hold on the dash. Breaking the tabs is very common when taking them off due to the age of the vent. To mount the assembly just press it on . The tabs will click in place and lock, there is a waffer spring washer that applies tension . As I said, if you have broken a tab, then you have to buy a new one. The dealer has them. BUT the center 240D vents are no longer made. So you either have to find a used one or use a 300D closeable one in place of the non closing 240D vent. Also the 240D center vent rubber duct is no longer made. However the 300D rubber duct does have reinforcement where the closing shaft goes thru the rubber. So it can be cut for the 240D.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car