Here I'm told that 3M makes best films for windows and they have ones that are perfectly clear but still block heat out as well as dark ones, those are mostly used for house windows, but window is a window no matter if it is in a car or house.
They don't sell those 3M special films to DIY guys, only available as installed.
I have not put films on car, but I have put on my house, bought cheapest ones, now after 1 year they are not dark at all, colour has disappeared in sun, so I advice to avoid films that are cheap.
With cars here we have quite strict laws about it too, front windows are not allowed, no windshield, rear window can be darkened but mirror is needed on passenger side if quite dark film is used, so I have not bothered as front would be more important, but as it is not allowed here I choose to sweat, makes me pick bicycle more often so maybe law is not so stupid after all
