a damp washcloth and a steam iron is all you need to get off the old adhesive. no risk to the finish, and it actually works. just plank the cloth over the area, and iron it.
a couple of thoughts to add, though: I did this to get the old numbers off my trailer, and it seemed to work 90%...there was still a few stubborn spots that wouldn't come off. or so I thought. I tried every chemical concoction I could find, short of paint stripper. nothing worked. So I broke down and tried some fine polish. the idea was to use the mild abrasive to scrape it off. almost immediately, the polish turned from white to black, and I realized what was happening. I was polishing the bare alluminum that was exposed by the old adhesive that had eaten through the clear coat.
so now I had BRIGHT SHINY ghost numbers, instead of dull, not-too-offensive-yet-still-visible ghosts.
as it turns out, the adhesive doesn't eat through the clear coat evenly. So it leaves a pattern that looks and feels like...bits of dried adhesive. What I was actually seeing was little bits of clear coat still stuck to the alluminum in a "dried adhesive" pattern.
So, the point is that the steam will remove 100% of the glue. it only "looks" like there is still some left. the only cure is a complete strip, polish, and recoat of that panel....which might actually be a cool effect
. but not worth the effort, imo.