Hi, Andy.
As britlude suggested, the rod is inserted from the front towards the rear.
Unless you have to, you really don't want removing the rear sash as you may create another issue such as wind noise, water seeping into the cabin, etc.
Please check the threaded section of the rod.
Sometimes, it gets loose and disconnected or the adjustment no longer suitable that it puts extra stress on the plastic bush/fastener that it will pop out or cause breakage resulting in the same issue as yours.
The rod goes into the upper plastic fastener.
The lower one is for the rod from the external key cyl hole.
This is the plastic fastener used for the rear sash rod.
Always keep spare when working on the rear sash as it's so easy for the yellow ring to fly away when inserting the rod back in place.
I normally use combination of two long nose slightly angled pliers to create leverage when dealing with this rod.
Kaz