Not sure how useful this information is, but I've heard that the only police that can actually give you a ticket for not having the tinted window permit are the Policia Transito. So if any other police are trying to shake you down I suppose you could "call their bluff" and tell them to go ahead and call the Policia Transito if they want to give you a ticket. Of course there is always the risk they will go ahead and do exactly that (and potentially impound your car)
This whole tinted window thing is very annoying in cases where windows are only slightly tinted, particularly on cars that come from the factory with some tint on the rear windows. The dark tint where you really can't see inside the vehicle at all is one thing, but IMHO it is an unjustified hassle in other cases.
Aside from the hassle of getting the permit in the first place, there is the fact that the permit is only valid for the specific names listed on it. I think it comes with two names standard, but you can pay extra for a couple more? Anyway, that basically means you can't (legally) lend your car to anyone else.
Or maybe that is a "feature" for people who hate lending their cars to others... it gives them an excuse to say "no."
I bought a car that had tint installed by the previous owner and I hated it. It was so dark and gloomy inside the car. It was oppressively depressing to drive, and it was also more difficult to see other cars in the rear view mirrors. So I had the tint removed.
However I left the protective layer that protects the glass from being broken and even just that gives a slight tint to the window that I've been told still requires a permit. Arg... so annoying!
Someone told me the fine for not having the permit is pretty big. The far worse part, however, is that they can take and impound the car. Having my car sucked into the Peruvian bureaucracy sounds like a nightmare scenario. I don't even want to think about the hoops one would have to jump through to get it back.
So if I didn't have the permit and were stopped I would definitely pay whatever bribe necessary to get out of it. I read some people complaining about paying bribes and how it makes Peru worse and I don't agree. Bribes are just a symptom of the real problems such as the lack of respect for police who only loosely apply the law because they are underpaid and have little incentive to do otherwise, and the inane bureaucratic nonsense that complicates so many aspects of Peruvian life and encourages people to take shortcuts. If you have some moral problem with bribes then don't pay them. However, if I can make my life a little more bearable in this extremely flawed system I will pay. I recommend aiming any righteous indignation at fixing the real problems.