according to parts books GM used the same seat 1997~2018 vans and similar in the smaller astro van
all of these use a lever to turn a splined shaft to allow the seat back to recline, this is located on the seat base on the rightside of the driver's seat, left side of the pass seat
I'll attach a parts diagram from a similar seat so you can see the location of this part #15-
#17 is the lever
you may need to remove the seat cover to inspect
you should see or feel a splined shaft
mike