Not sure how it would work through lexan or how the blades would do on it either, but the usual way these are installed is you insert that threaded part through a hole in either the roof section above the windshield, or in the cowl area below the windshield.
You will notice that the threaded part of the shaft is long, so what you do is you run one nut down all the way then put a washer and then a rubber washer on. Then you slide the whole assembly through the hole in the car, put on another rubber washer, a regular washer, and finally the second nut.
By adjusting the two nuts in or out you can move the motor closer or further away from the body, and when you get it where it needs to be you drill a hole where that outer hole is on the body of the motor and put a screw into the car. This acts to keep the motor from twisting as it runs. The final thing to do is hook up the wiring and put the arm and blade on in the correct position so the sweep will clear the area you want it to clean.
I think the blade could possibly scratch Lexan, and really, Lexan is not a suitable material for a windshield. I know some people use it, but approved safety glass is really the only legal, safe way to make a windshield.
Don