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IIRC, the vent goes in first, then the regulator. Make sure the regulator is retracted somewhat.
I'm old, but I thought I did the vent first without the regulator so that I'd have more room in there. Then after installing the vent/channel LOOSELY, and inserted the regulator and got the screws in a few threads. I kept it all loose to get some extra wiggle. I think side glass was last.