I had success with these 2 approaches (recommended by 3M btw), assumption is car has been washed & dried before doing any of this. Also, both of these will remove any wax or quick detailer and even mess with ceramic coatings, so I put down a w-i-d-e blue tape border around the strips. You want to work 'wet' but not rolling off the car wet. Use MF cloths only or a very, very soft brush if absolutely needed. To remove the cleaner, I used wet bath towels to wipe down the area I was working, then MF to dry. You may need to go over them more than once depending on how bad they are & how baked in the stuff is.
Based in type/degree of contamination, use one of more of these solutions, in the order shown, to remove any build-up:
1. Hot, soapy water solution (Dawn is best for this - not too soapy then be sure to rinse well)
2. Isopropyl alcohol (IPA) and water (ratio 2:1)
They also mention Simple Green but I found that to be streaky. Key is NEVER rub hard/scrub or use any sort of abrasive brush or you will permanently mar the surface. Once clean & dry, I started applying 303 with a MF covered sponge going up & down, then back & forth & finishing by lightly wiping in single swipes top to bottom with a fresh MF sponge. Mine needed multiple, lite applications over the course of 3 or 4 days to come out 'even'. I put that finishing sponge in a baggie (it has a small amount of 303 on it) and use it for a quick refresh after washing.
Pro tip - go to one of the websites that sell vinyl wrap and order some 1' x 1' samples (3M Carbon Fiber series) or even a few feet of the stuff. You can use that to practice on and try different coatings/cleaners etc. to see what works/looks best !