I had the same problem on my Mazda. There is a catch at the bottom of the rear doors that needs to be engaged; once you shut the rear door, pull on the door near the bottom and make sure it is closed. Drove me crazy until I realized what was happening.

Good luck.

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2002 Mazda B4000
4.0L V6 SOHC, 5-speed auto, Cab Plus 4WD
2015 Subaru Legacy Premium
2008 Subaru Forester X (Gone, bad trans)
1999 Mercury Mystique GS
1967 Mustang, 289 auto