I know what your talking about.
Like if you try to clean the inside (tint side) with a light colored towel,the towel will have black on it right?
There are 2 things that I know of that can cause this,cheap tint(not 3M or Lumar) and if someone uses the plain windex or any cleaner with amonia in it,as the amonia will eat at the tint.
From my past experiences the only way to fix is to get it re-tinted.
Those 2 things happened on my 01 GT,when it was at the repair shop from when the drunk hit it,they cleaned the window with windex and within a week it was just like I said,so they paid to re-tint and did some cheao stuff,3yrs later when I get rid it that cheap tint is hazed again.But it did look good on the outside.I think the heat really affects this,as my GT was blackand would get really hot inside,and if I remember right your Mach is black too?
Around here they charge about $80 to do the back window if it has heating elements in it.