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.