To install it, the top and bottom slider door brackets need to be removed one at a time, so the bracket arms can be located within the inside of the weatherstripping rubber loop of the door opening.
No problem right? Loosen three lower door bracket bolts first.. slip the bracket under the rubber.. and then loosen the two Torx screws holding the upper bracket..
All of a sudden the backing plate falls down the door cavity.

As it's a cargo van, an access plate is held on by a few metal screws.. no dice.. the access plate allows access to the side door beam and not much else.
there's an oval rubber plug near the middle bottom of the door. i got a giant magnet and teased the backing plate from the inside front of the door till I could grab it in the opening with my smaller flex magnet pick up tool.
Here's my imagined diagnosis of what happened. Back around 1999 when a welding robot was assembling my Astro in Baltimore, a glitch must have happened to cause only 2 of the 4 backing plate welds to be performed. ( or an accountant ordered engineering to eliminate the 2 spot welds.)
Whatever the cause.. over the last 20 years of door use, the backing plate has been flexing slightly.. (see tiny cracks)... The thin backing plate also contributes to the flexing of the door jamb sheetmetal as it isn't wide enough to distribute the forces of the door slamming inward while latching shut.
but because the torx bolts have been holding the upper door bracket all these years.. it didn't really matter until today.. when I expected the backing plate to remain in place when I removed the bolts.
My solution plans will be to drill the spot weld remnants from the backing plate, find my fish tape tomorrow and fish the plate back up into position in this skinny boxed in section of the door shell, and use some flush nutserts or rivets to secure the backing plate in position.