Here's how the splines fit into the window frames. they are held in by a screw on each side. These screws can be REALLY difficult to remove. We found a bit brace helps a lot as you can apply your full weight to keep the screw bit from slipping. Also apply some acetone/ATF mixture, or the equivalent to PB blaster to these and let them sit first to try to loosen them up. The screws are much harder than the aluminum frame, so if you have to drill them out, it's a mess. You only really have to get out the side that faces to the center of the window as that's where most of the water comes from.
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