Tinting a side window without removing the gasket or panel

Method 1: If removing the gasket is not possible, there are a number of possible solutions. The first thing you can do is get someone on the other side of the window to pull on it with a suction cup in order to create a gap between the glass and the gasket. Then use a card to slip the film under the gasket. 

Method 2: Use a gasket wizard. These tools are specifically designed to make slipping gasket under gasket easier.