This may be of interest.

My wife sent her 6 wheel truck for its annual registration check and it was failed due to the window film she had recently installed to keep her driver comfortable. Kind hearted that she is. Thats 8000 THB down the drain. It was not a question of the darkness of the film. The rule is that no film is allowed. However I do not know if this would include one of the clear window films such as this product from 3m:

http://solutions.3m.com/wps/portal/3M/en_US/Window_Film/Solutions/Markets-Products/Automotive/Crystalline_Automotive_Films/

Has anyone installed a clear window film in their car or truck?

For cars window film is accepted, think allowed as well but I'm not 100% sure of that.

For minibuses licensed to carry passengers, so yellow plated, I know window film is not allowed and it will be rejected (can be overcome with a donation though).

Would they even notice clear film?

Shaded film works by reducing the amount of light going through....that means that you can SEE LESS. Thailand seems obsessed with excessive amounts of dark film. The benefits are in reality negligible, except of course that other drivers can't see who the idiot was who just made the driving blunder.

For cars window film is accepted, think allowed as well but I'm not 100% sure of that.

For minibuses licensed to carry passengers, so yellow plated, I know window film is not allowed and it will be rejected (can be overcome with a donation though).

Yes, my wife said that most drivers/owners must be paying to pass the truck.