Tell them of the risks, if it's something pricey, have them sign a paper agreeing that it "may" cause a problem with their paint but they are aware of the risks and will not hold you accountable.
Soap and water.... wipe the surface dry. feel wit your fingers if the surface has a grainy feel. if anything use a promotion cal and see how well a sample strip sticks. Sticks good, peel off sideways (not away from the surface), then apply your graphic.
I've lettered a lot of oxidized surfaces, I've lettered on freshly painted surfaces a few hours after coming out of a paint booth. Never tracked a vehicle, never ripped a chunck of paint off, and sure the heck, never wiped any type of paint thinner products on soft paint or oxidized paint. it's suicide. You may have gotten away with it in the past but that's probably because whomever painted it did a thick coat. I wouldn't push my luck with all these other crazy options either. I saw Windex mentioned, if it's worked, you're lucky, because Windex & other glass cleaners usually contain ammonia which causes issues with vinyl adhesives.