Okay so replacement mirror elements come in two types---first is the OEM replacement which includes the mounting or backing plate--like this: https://www.ebay.com/itm/2008-2009-F...53.m1438.l2649
The other less expensive version has you removing the old broken mirror bits, cleaning the backing plate of all remaining adhesive before using a Burco mirror-only mirror piece. A few photos of the pieces involved in that process:
Backing plate with damaged mirror removed, awaiting new glass:
That piece ^^ snaps onto the power motor base:
Personally its best to have the above piece in place with the entire mirror mounted to the van before installing the replacement mirror element. This assumes you're not using the OEM part where the mirror and backing plate come already installed.
(That image shows a backing plate I've prepared and modified to accept a heating element, the squarish holes do not exist in factory original parts.)
That's a quick over view of how the mirrors are built. This replacement process works for both manual and power mirrors. This is aimed at OEM mirrors and not the aftermarket types.