NOTE: This modification was done on a 1998 Holden Suburban from Australia which is Right Hand Drive. The required modifications are likely very similar or identical to a US vehicle, however be aware you may have slight differences in your vehicle.
Firstly....I had to shorten the shaft which mounts the visor to the mounting bracket.
This meant grinding the 'folded over' top off the shaft.
Next I cut the 'hole bosses' down so the mounting brackets would sit flush on the ceiling.
Next, I trimmed about 10mm off the plastic on the mounting shaft so the plastic coating sat a fraction below the edge of the hole in the mounting bracket.
Then, I cut an 8mm thread on the shaft.
Next step was to install a nut and then cut the excess threaded shaft off flush with the nut.
I tapped a 12mm thread into the mounting holes in the plastic mounting brackets so as I could insert a 'step' for the mounting screws to rest on.
I drilled a 8mm hole through the centre of a 12mm bolt and then cut slices off it.
I super glued and threaded these 'slices' into the 12mm tapped holes in the mounting brackets.
So...that's the visors prepared.
Next....mounting them to the ceiling of the Burb.
OK...after checking the position of the new visors in relation to the recess in the ceiling of the Burb....I figured I could use the original mounting hole for the smaller 'clip' bracket.
I pushed the OEM clip out of the way and drilled the hole out to 9.5mm and installed a 6mm nutsert (if you don't have one of these tools...get one!).
I next clipped the visor into the bracket and held it against the ceiling. I marked the holes through the bracket on the other end.
I then removed the bracket from the visor and marked the ceiling properly.
I cut the centre out of the hole with a sharp knife and stuck my finger in there to make sure there were no wires hiding in there.
Next I drilled one hole in the ceiling with a small pilot bit.
I then cut around that hole and drilled it out to 9.5mm and inserted a 6mm nutsert.
I mounted the visor up and drilled the next two holes 'through' the bracket so as to get the exact position and correct angle. Then I inserted the next two nutserts.
The ceiling has 2 inner skins in some sections (2 of the 3 holes) so you need to drill through both. Be careful not to go too far. If you can see daylight through the holes....you've gone too far! Lol!
Next I fed the wiring through under the ceiling trim, plugged the mirror in and mounted the visor up.
I just need to make a cover plate to go over the old holes. I'm thinking either polished stainless or aluminium OR grey plastic.
Pretty much done!