I'm not familiar with '46 trucks, but on the later trucks (47-55.1), the seat back cushion is held in place on top with two brackets and screws at the top of the seat frame. Take a look up there.

[on edit] See the attached pic of the seat back on my '51.
Attachments
Seat Bracket.jpg (205.54 KB, 143 downloads)

Last edited by klhansen; 12/31/2022 9:56 PM. Reason: added photo
Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.