Hello, I actually used be a way into the minitruckin' and custom car scene here on the West Coast. I have meet and gotten drunk with the editor of the Japanese Magazine 'Truck Trendz' when he was in Salem, Oregon for a show and from what I have heard from him there is a pretty big lowrider/custom auto scene in Japan. LowLifeVideo out of SoCal has been to and filmed shows in Japan, along with MiniTruckin' Magazine.
You could always email them or post on their board for question.
TruckTrends
or these guys here..
http://www.bluepanic.jp/main-top.htm