I removed the mirror
button by heating it with a butane lighter (someone else's suggestion, Wally
probably) and periodically trying to twist it a bit. Eventually it gave. I was a
little nervous about this, as I was envisioning the temperature gradient across
the windshield, wondering at what point it would crack . Maybe it's not a
problem, but who knows? Whatever you do, don't try to use a plastic button and
crappy silicone-based glue like I did the first time (I can't remember why I did
that, but I think that was the Donnelly-recommended option at the time). That
didn't work at all; it would jiggle all over the place in the Kansas heat and
was probably destined to drop off . The classic metal button and drop of mirror
glue was the way to go and is holding strong to this day.
'90 928S4 black/black
It's the difference in temperature between the glass and the metal that causes the glue to stress. The glass is laminated so the temp applied to the outside isn't necessarily transferred to the inside layer of glass. Nonetheless, a cool/wet towel on the outside might slightly reduce the risk of damaging the glass. On the inside, make a heat shield out of aluminum foil. After that-- lots of heat quick on the metal base. The little butane torch is fine, and you could probably just use a heavy-duty heat gun so long as the foil shield protects the glass. Anyway, faster is better. Then, with vice-grips, twist the base loose, don't pull on it. Pulling is the maximum risk of damage to the glass, so twist to break the bond and it will fall off.
'89 S4 Auto, black
Acetone? Just don't let it drip on the dash.
yes, use a cheap $1 plastic painters drop cloth. Good catch, I forgot to mention that.
I put the acetone on the windshield above the metal block using a cotton ball. Keeps the drips down to a minimum. The acetone will suck into the gap by capillary action. Unless Porsche used silicone, it should work.
I tried acetone and many other things - mine was quite immune...
Ed told me his trick. When I replaced my windshield with a stock unit that came with the button attached - the installer showed me the trick - he had a pair of needle nose channel locks on the tab and heated the metal base of the tab with a roaring butane torch for about 2 seconds - twisted the tab and it came right off... No fuss - no time for the glass to heat. Dr Bobs idea of a foil heat shield is a good idea too for the less experienced - this guy was good though - it was off before I hardly realized he had started!!
Here's how I removed the mirror and mirror button from the windshield on my 1990 S4.
The factory rear view mirror fits into a slot on the windshield metal button. It's just a wedge fit, so you can work the mirror off the windshield by hand toward the rear of the car, rocking it slightly sideways back and forth as you work it free. No locking mechanism to release, so it's pretty easily removed.
To remove the metal mirror button from the windshield, I warmed up the windshield glass (both inside and outside) using a hair drier, but only to about 80degrees F.
Then I heated the metal button with a common butane lighter (Bic). Just apply a small flame for about 5-6 seconds. It doesn't need to be very hot, just hot enough for the button to feel hot to touch, but not hot enough to burn. While the button is hot, I used a long flat bladed screw driver with moderate force to 'rotate' the button and let it pop off. You should feel the adhesive flex slightly giving you a hint that it will come free with maybe slightly more heat. Don't try to pull it off, let it pop off as you rotate the button.
Here's a photo showing the button removed with the adhesive remaining on the windshield. You can also see the grease marker that I circled from the outside to mark the 'old' location as reference for the new Gentex button that I will install later.
Besides needing to removing the old adhesive, I'll also have to remove the decal about the air bag feature, as the mirror new hardware fits over this decal.