Interlocking original style seal or one with a lock strip? Is it one continuous (fused) loop or single rope style that must have the ends trimmed to fit?
There are two basic techniques. One requires placing the rubber on the glass first, and using paracord, pull the lip of the deal through the window opening. I've never tried this with fiberglass. the rough, and more fragile edge of the fiberglass may not work well. (See attached explanation)
The way I have done it is by placing the seal on the fiberglass first. If it is the rope style, make sure you have the seal trimmed to fit perfectly butting the ends at the bottom center of the opening. Place the glass into the seal as much as possible, starting with the glass in the bottom channel. Work the tightest radius corners first. The four corners, especially the last one, will be the most challenging.
Then it's a matter of coaxing the rubber lip from behind the glass to the outside. Do this by gently pulling your spatula or pallets knife between the glass and the rubber, and gradually coaxing and gently prying the lip from under the glass, while keeping firm pressure against the glass. Once the glass is all the way in the channel, the process is pretty much the same in getting the interlocking strip mated.
Tools:
A thin and narrow plastic spatula that will withstand a fair amount of pressure without flexing too much or breaking. A sturdy steel palette knife with rounded corners is also good, but requires more care near painted surfaces (in case you slip)
Find some actual rubber lube if you can, otherwise some liquid dishwashing soapy water solution will do. Black Magic Protectant works better.
Patience, patience, and more patience. Expect to work up a sweat. An extra pair of hands to hold things is advisable too.
It's a good idea to tape around the edges of the area your working. If you tool slips (which it will), you'll have a better chance of not damaging the finish.
Do it on a warm day in the sun or in a warm environment; the rubber will be more cooperative.
Last, but not least, consider having it done by a professional. They make the job look easy and it takes the responsibility off you if you break the glass.
