I have 3M clear bra on the rear quarter panels behind the rear wheels and after three years they were no longer perfectly clear. Since I was about ready to get them replaced, as well as getting some installed behind the front wheel well and across the front of the hood, I figured I had nothing to lose.
I used a soft foam pad on my PC with Menzerna Nano and it took out all the scratches but it left a little haziness. So on the other side I used Meguiar's PlastX and it left some of the deeper scratches but made the undamaged portions look great. So I went back over the Nano side and afterwards the haziness was gone. I would probably just use the PlastX only since the scratches were not very noticeable anyway.
I am going to get them replaced anyway, so they will all look the same. But it is worth trying some different products on your to see what works best. Just don't use anything too abrasive and you should be OK.