Let me know if this doesn't work for you, and I'll go in-game and check. My guess is that you probably aren't looking in the correct shop.
Also, engine neons require the highest level engine upgrade, and trunk neons require the highest level trunk layout upgrade, so that may be why you're missing some of them. They are found at the same shop as the underglow neons, in the same menu.
GT in profile - send a note with anonymous friend requests. Gameboy Micro: the most portable hand-held gaming system to date.