You can add a Nutsert; use sheet metal screw, screw/nut from inside; or leave out. I've had some on two of my Vanagons for 10+ years and they haven't fallen off yet.;-)

BenT
sent from my electric manual mirrors

On Jun 18, 2013, at 4:15 PM, "Andrew R." <a.richar@GMAIL.COM> wrote:

> Hey all,
> 
> In an effort to get droopy mirror syndrome behind me, I picked up a pair of
> power mirrors for my manual van.  Not intending to wire them up though.  My
> question is around mounting.  I know there is a third mounting hole for the
> power mirrors to use.  Do I need to use it?  Has anyone been down this
> road?
> 
> thanks,
> Andrew
> '87 GL