On some cars,there is a plastic bit about halfway up the diagonal bit on the front side windows.Whats it for?
like a little rubber tab? - I believe it’s part of the rubber window seal and is useful for holding on to the assembly when fitting the glass.
You mean near the edge of the windshield? My Volvo has a plastic tab there, I hear it’s intended to hold up parking permits or tickets. I use it for that purpose anyway.
I believe the OP is talking about the plastic bit on the diagonal edge of the window FRAME, not on the glass itself.
I believe that the purpose of the plastic bit is to “catch” the edge of the glass and funnel it into the groove as you roll up the window. Otherwise, if there was any slight misallignment, the edge of the glass might smash into to the side of the channel, ruining the weatherseal.