I like what I think are practical mods.

Better Fuel Filtration
http://www.peachparts.com/shopforum/diesel-discussion/225004-lower-micron-fuel-filter-mod.html

Bypass Oil Filter
http://www.peachparts.com/shopforum/diesel-discussion/215593-bypass-oil-filter-setup-617-952-a.html

Air filter/intake
http://www.peachparts.com/shopforum/diesel-discussion/280376-washable-air-filter-intake-mod-larger-breather.html

Manual Cable to open unlock trunk
http://www.peachparts.com/shopforum/diesel-discussion/304114-manual-pull-cable-wire-open-trunk-123-a.html
I was in the process of installing a nicer Trunk Cable setup up when I found that a Rear Cross Member under the Car was cracked. So I have yet to finis the new Trunk Cable Set-up.
I mounted it to the left Front of the Drivers Side Seat. See Pic.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel