the holes are the same on the drivers side, even a little bigger.
maybe the original nutserts came out, or were removed when they failed and started to spin. A common issue with old nut-serts it seems. Yet some manage to last the life of the vehicle.
I guess these mirrors get loose alot and the PO put some oversized sheet metal screws in them.
Also very possible.
How the hell would I drill a square hole for the nutserts??
You don't. The square hole comment was regarding the black plastic inserts/anchors/thingies.
Nutserts take a round hole just like any normal fastener, and are crimped into the hole like a pop-rivet. Using either a specialized tool, or a long bolt with nut on it to pull the nutsert up and lock it in place.
I do like the idea about just putting some nuts and maybe lock washers on the inside of door. Yeah if they got loose I would have to take the panel off but it wouldn't be that bad I wouldn't think
You're right. Not the end of the world. But the previous suggestion about the "nut plate" is even better I think.
If you don't have taps and dies (they're not that expensive if you only need one or two common sizes, but a whole cheapy kit is probably under thirty bucks still) you can still glue them in place.
A bit of steel plate the right length to allow it to span both holes, with holes drilled and threaded to match, can be glued to the inner panel of the door. This "glue" could be anything from an epoxy to silicone sealer, to panel bond, to double-sided tape, to whatever you have laying around. The glue does not have to do more than hold the plate in place the next time you remove the mirror for some reason.
Even silicone makes an outstanding glue if it's given enough time to cure. Silicone it in place, then it just sits there possibly for years. After that, it's unlikely that just unscrewing the mirror will dislodge the plate. So you won't even have to go back into the door.
Same would hole true if you don't have taps and just glue nuts to the inside of the new bit of metal. The nuts themselves are too small to get a reliable glue bond by themselves, but if "glued" to the metal plate and the metal plat is adhered to the door panel, it should work very well.
And while you're in there you can deal with all the rattles that develop over time!
Anyway, just some thoughts on different methods. I finally bought a Nutsert Kit for just this reason. My mirrors were loose and attached differently, then I changed mirror designs and have always wanted nutserts for various jobs on the vehicles.
Paul