As Mardy says, later model L300's had two dimples along the underside of the screen to act as a drain - allowing water running around the
inside of the screen rubber to escape. A lot of the L300 I have seen without these dimples have got
some degree of rust in those locations. I have yet to see an L300
with the dimples suffer from any rust in those areas?
If you do want to sure it, the removing the rear screen is the only real solution (as shown by Trapshooter) but whatever you do...DO NOT push on the window or add any pressure as you pop the seal - this could well cause the screen to break! Gradually work your way around the seal from the inside feeding it over the lip (best to start across the top, down both sides then across the bottom from inside the vehicle) while someone holds the window from the outside to prevent it dropping. They
must not put any pressure on the glass - simply support it as it comes out.
Trapshooter is spot on about refitting - run some string/coated wire around the inside of the seal and squirt some washing up liquid around the rubber to make it more slippery. Pull steadily and carefully and if the string gets caught, don't start pulling hard because you'll only slice through the rubber!
The other thing to consider is doing this in a warm environment/on a hot summers day! The rubber will be a lot softer and more maleable when it is warm - which will make the job a lot easier and less chance of tearing/splitting the seal.
One last thing - it might be worth borrowing or even buying some suction handles for your helper to hold the glass properly. This will make his job a lot easier and minimise any pressure he may put on the screen. You can buy then quite cheap from DIY stores....
