Given the (in focus) green door in the back, i presume your asking about the rubber profile that seals between the upper door and body, at the roofline?
The one that has a plastic backing that does not really like to be folded in the slit?
I removed it only a few weeks ago, and did not yet try to get it in, but i wondered about the reinstall as well.
Could you slide it in, all the way from the end?
Other members will chime in, as it is an often replaced part during a complete overhaul.