On the power mirrors for the NA, both my 93 and my 95 were the same removal process to attach the snap plates. First, take your window down. Then twist the mirror perpendicular to the car, there's a large spring attached underneath in the center that allows you to do this. That will allow you to access one of the phillips head screws. Then you twist the mirror back to its original position and keep going to twist it around the other way. Now you've exposed the other screw for access with your screwdriver. Remember, this is for POWER mirrors. I don't know about the manual ones.
Once you get the screws out (and this might be a chore, get the correct screwdriver or ask Don about that impact driver) place the mirror gently inside the car. You took the window down, remember? Don't place it on the outside of the car as it might damage the paint. It hangs on that cable, but if you are anal, support the cable. (I actually had to remove the mirror on my Merlot to replace it and that connector inside the door was a bear to release. Gently hanging the mirror by the cable shouldn't pull it loose.)
Check out this forum and run a search of the site for more info. You may have to register first - free and simple. A good source of MX-5 stuff.
Forgot to say, welcome to the MX-5 obsession and enjoy your car - it's everything the MGB should have become.
