Mylar scratches easily and degrades rapidly in prolonged sunlight (ask any windsurfer).
Like thousands of others, I have heat rejecting film applied to the inside of my front windows. I went with 3M's version and it made a very noticeable reduction in heat - so much so, that my stock a/c is too much for even the hottest desert days and I now run with the heater cracked open when the AC is on lol.
From an engineering perspective, I doubt there would be much if any measurable benefit in placing the reflective surface on the outside, since the objective is to reflect the heat back outside From what I have see, very little of the heat trapped in the glass layer is transferred to the inside.
I do like the idea of putting reflective film on the exterior metal parts - although you run the risk of blinding oncoming drivers/riders if the sun is at the wrong angle. you get this with highly polished aluminum trucks/trailers.
Quigley Transit mid roof, work truck, equipment hauler and occasional camper