What can cause this?

I keep thinking its contamination, but I do triple passes with cleaner and razor blade and still get it.

If you are talking about the white/silver looking crap at the bottom left of the first pic then it is contamination that is wicking back up from the rubber gasket when you are installing. Those old rubber gaskets are nasty on anything you might get! Lots of ways to work it better.

Looks like a jeep cherokee window....pull that bad boy out and lay it on...they are simple to pull and install.

Not sure what vehicle that is..but just the fact it's rubber of that style..I'm guessing some old vehicle. Trim the inside rubber down a bit evenly (you'll also remove much old contamination this way.

Flush many times with solution around the perimeter and wipe away. Finally when you go to install film, spray the film generously with mounting solution , the glass only a light mist keeping AWAY from the perimeter..you want to keep that dry..if you don't..it will only dislodge more debris. once on , squeegee and towel many times the perimeter quickly to that contaminants do not backwash into the film.

maybe a little rubbing alcohol on a microfiber and wrap around a card and run it around the edge.

Thanks guys, I should of been more descriptive. It only happens around the edges and its a 1992 Jeep Cherokee!

is that a heater element on the back glass? if so, swap the razor for a white scrubby or you might damage it.

pull the window out. cake to do and it's an easy rope in. then no contamination. you can pull the quarter glass windows out for doing them too.

Pop them out and you can get the film behind the gasket. No contamination and No light gaps.