Remove all of from the car ,Lay the glass on a table flat ,Use a block of wood flat against the glass at each end of the channel , hit the wood with a hammer even both ends ,
I have used a 12" - 16" 1"X2" successfully - As Flathead Ted says - place wood on each end and tap out - I rest my fist holding the piece of wood on the glass - keeps glass from moving - and tap til it breaks loose alternating ends as necessary. Good luck.