I'm not familiar to car tuning shops. However, I can tell you that there is quite alot of stuff you can do around Tokyo, mainly shopping.
Not sure if you know this already, but there is a train called Yamanotesen that makes routes around Tokyo (if you're actually at the train station, it's colored green). Basically you can take that and go around Tokyo, then get off at any stop. After you get off the train, you'll have to find the exit. *IMPORTANT* make sure you know what exit you to go out (they have west, east, north, south exits). If you don't remember, you'll have to make a huge loop around the town to get back. Pain in the arse. Also, I wish you can read Chinese or Kengi, else I could list tons of place to visit around Tokyo (I can't remember what they're called in English that's why..).
Though, one place you MUST visit is "Akihabara". Akihabara is known for the "eletronic street" or "high tech street". After you get off at Akihabara taking the Yamanotesen, follow the sign to "Eletronic Street". There, you'll find building as tall as 8 stories high building, and any small shops there that sell high tech electronics (such as PDA, CD player, comptuer equipmetns, car audio, MD player, stereo, TV etc..). If you're first time there, you can pretty much spend a day there looking through each store. In addition, if you're planing on buying anything, bring your passport because some shops will give you duty free :>
As for car audio, if I remember correctly, I think they're just across from the train station exit, they're inside a narrow street (kinda like flee market looking). Though, I'm not sure if they'll work on our car or not.
Sorry if I wasn't as helpful as you expected, but if could read Kengi or Chinese, I could type it out some other places that you can visit. There is also a street full of stores that sell clothes, and name brand stuff. Lastly, I'd go to bookstore and look for some tourist guide for Tokyo, it'll probably give you a better idea on where to have fun.
P.S. I think all the car stuff would be at Yokohama. I think the main Honda Corp. is at Yokohama.
