Door check strap. It can be accessed by removing the interior panel. I didn't even completely remove mine when it failed BECAUSE the typical failure mode is not 'wont open' but WONT CLOSE which is way more problematical when your away from home at the side of the road with an open door. So, fix it soon.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13