there's 2 types of flag mirror for the 944, i guess a 87 928 will have the later one;
http://www.clarks-garage.com/shop-manual/body-10.htmfiddly buggers if you have same as late 944 then you can see the notches the guide above refers to by adjusting the mirror fully upwards. I did it from the front between the glass and the frame after unsuccesfully poking around through the hole in the bottom (fnar) for about 15 minutes. The plastic ring rotates about 1/2 an inch then the mirror lifts out fairly easily. Couple of wires are socketed on the back for the heater element which just pull off.
i think theyre all heated initially, the glass came loose in mine too last autumn, the element is in the plastic plate they sit in.