How is the clear plastic, easy to see through, good quality for vision?

Yes it is made of a high quality marine grade vinyl.

I have an Icon20 and wonder if you carry any covers that would fit other than the one with the hinged door

Yes we offer a great Track-Style for the Icon I20 located here:

https://acegolfcartcovers.com/products/3-sided-track-style-enclosure?variant=41306180026536

Does this cover fit a rear seat cart with an80'' top?

It does not. For an Over-the-Top on 80" Roof this is be the correct product: https://acegolfcartcovers.com/products/golf-cart-driving-enclosure-for-4-passenger-roof?variant=39724710690984

Will the back of the cover roll up so that we can access our clubs easily?

Yes, it is great for club access.  The back cover has a 2 U Shapped Zippers. One on the top to access clubs and one on the bottom to access bag (balls, tees, etc.).

Can I use this if I have side Mirrors?

This enclosure works great with mirrors. You can either remove the mirrors and reinstall through the fabric or secure the fabric around the mirror as you attach to the front roof supports.

I would use the 3 sided cart at public courses. Does it fit most standard carts?

This 3-Sided Fitted "Over-The-Top" Golf Cart Cover is not the correct style for your use.  It is made to fit the exact cart model and is not a universal fit.

If you are looking for a portable cover to use at public courses, please consider the Premium 4-Sided Universal Golf Cart Cover. This fits almost all standard golf carts and has a removable front window since most golf course carts have windshields.

4-Sided Universal is located here: https://acegolfcartcovers.com/products/champion-four-sided-throwovers

Does it work if you already have the club canopy on club car tempo2021

It would be a tight fit and you could not use the Canopy. We suggest the Track Enclosure, this enclosure does not affect the canopy.

https://acegolfcartcovers.com/products/3-sided-track-style-enclosure?variant=40269731889320

The 3 sided cover I assume there is not a windshield? I have a windshield installed so require only sides and back

Thats correct. The 3 sided over the top cover does not covers the front windshield. It covers the two sides and the back.

Will this fir a 1991 Yamaha G8 cart?

Unfortunately, it would not.

I prefer the look of the Utility Box/Rear Facing Seat cover, but I have the regular golf setup. Will the Utility Box/Rear Facing Seat cover still work if I put it between the seat and cargo basket?

Yes

You mention removing the front windshield on the fitted 3 sided over the top golf cart cover! If it is 3 sided why would you remove the windshield? I hoped the three sided cover would allow me to see through my clear windshield?

The 3-Sided Over-the-Top Golf Cart Cover does not have a front window. The golf cart windshield may need to be removed and reattached for install. You will be able to see directly through the golf cart windshield.