Exactly. My back windows are 5% on top of the factory. They are dark. My very back window still looks blacked out, but is still able to see out of at night. I dont regret my setup one bit.

 

My front and back windows read out at about 1.8%. Check my gallery for some pictures. BTW my front windshield is also tinted at 35% on the whole window.

 

I just rcently got my windows tinted on my 1990 f150. I got 35% on the two side windows and 15% on the back... i regret getting just 15% on the back now cause you can see through no problem. I would definitely go 5% in rear windows imo.

 

i only have 20% on my front 2 windows, and factory on the back, and still have trouble seeing out at night when im backing up. but i think if you do like boilerup suggested and mount some extra reverse lights, you would be happier with the darker tint. i plan on adding some lights soon. i have 10% all around on my Grand Prix and love it