Tinting back window has no effects on defrosting. My previous car before FRS had back window tinted for 9 yrs and no issues as well with defrosting.
I did have a issue back in the 90's with my Honda Civic Hatchback because the stupid dealer had inexperienced workers do my tints.
They cut into the rear window grid, those lines on the glass. When that happens the window has to be replaced.
Have your tints done by a reputable tint shop.
Been told the silver metallic looking tints can interfere with power window switches. Get ceramic tints and call it a day.
