I wouldn't say "decline" as much as I would say "shifted paradigm". I think a lot of the kids are discovering that a loud muffler, big wheels, a wing, and splashy graphics gets them a lot of undue attention from the local constabulary and from thieves. They seem to be spending their money on high-end audio and video instead. I dunno, maybe its just me...

In interesting factoid: My neighbor is an insurance adjuster for a large US-based company and recently he has had several claims where the stereo stolen from the car was worth more than the car itself. They actually will total the car in a situation like that. So a word of warning, if you have expensive aftermarket parts on your ride consider getting a declared value policy.