I know you've tried this...

Start with the Seat Bottom angled at a 45 degree angle (with the front up-most)
[You are trying to "Scoop" the Bottom under the lower portion of the backrest]
As you are lowering the front of the Seat Bottom...continue pushing the front
of the Bottom rearwards and finally press the front of the bottom down to hear the latches "Click" closed.

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'84 300SD sold
124.128