What about new plastic panels they produce now to cover the a pillar.

This is what I have on my 65 coupe.
Appears there is a channel that could be used inside the A-pillar.
I tried from both directions with thin safety wire without success.
Might have to enlarge the existing or add an additional larger hole to get wires through.
Either way, not possible without minor partial headliner removal.

Another option I see is going across the headliner rods and to the quarter panel C-pillar and across door sill plate.

I’m not familiar with layout of Fastback C-pillar structure.

Funny you mentioned this map light mirror, I have one from a 2001 Mustang convertible.
I even have the visor lights and wiring.

If you use BB’s route around windshield trim, you could use SEM color coat if needed on where wiring is exposed to make it disappear as needed/if needed.

Nos681 – thank you for your picture, it is helpful to see where the A-Pillar channel actually meets the top windshield channel.  I can see that even if both these channels are open to one another, making that turn would be difficult to accomplish.  The picture attached is the OEM headliner in my 65 (looks like I have some detailed cleaning to complete!). 
The mirror I have has the map/courtesy lights mounted in the mirror assembly.
How did you run the wiring for your 2001 mirror?
 
True74yamaha - you are correct, these covers would be the easiest way to accomplish, I didn’t realize these were available, I think I'll be going that route, thanks for the info. 

I'll use the mounting bracket for the convertable mirror and glue to my windshield. 
I had an old 65-66 Mustang mirror that I've cut the mirror support bracket off, while mantaining the sun visor support portion.  I finished off the 'cut' portion with black piant the will tend to 'hide' it against the black headliner. 

Nos681 wrote:

You took the easy way. 😂

Yes, I did, what are your plans? 
 

In looking it over, as the new mirror is thicker than the original, without trimming back the original bracket the new mirror would stick out further by about 1”.  I’ll look it over some more, and let you know if I come up with anything..
How do you plan to run the wiring?

I’m staying with the original visors.  I checked the width of the convertible mirror for interference with these visors and there is none.  I plan to mount the convertible mirror in the same approximate location (distance from the windshield) as the OEM mirror.
If I didn’t have the headliner already installed, I would plan to run the wires through the top channel and down the A-pillar channel.

Bullet Bob wrote:

In our 66 HT I considered running the wires through the header channel and down through the A pillars.  Wouldn't go...blockage at the top of the a pillar.  That's why I ran things under the rubber flange of the gasket.  Invisible and works fine.  Easy too.
BB1
 

BB1 - thank you for this feedback, I will struggle no more with the "A-pillar" routing decision.