Hello and welcome to the forum!
I’m not hugely intimate with the Japanese police cars (thank the gods - although I have been pulled over once, not fun) but I am a Wikipedia fiend! The Japanese Wikipedia page on Patrol Cars gives a mind blowing amount of information about the cars.http://ja.wikipedia.org/wiki/%E3%83%91% ... B.E3.83.BC
Doing my best to muddle through that, here’s a short (and most definitely incomplete) list of some of the extra equipment on Police Cars here:
Red light bar (some vehicles have an electronic scissors jack underneath to raise the height of the light bar for added visibility)
An added amplifier for the sirens
Speed gun (radar gun)
Car locator (reports the location of and condition of the car to the station)
Drive recorder (Russian dash cam essentially)
Passenger side door mirror
Passenger side navigational mirror (additional mirror for the partner to look out of)
Siren switch on passenger side floor (apparently they can switch between a foot operated mechanical siren or the electric siren… maybe, my Japanese isn’t that great)
Electronic noticeboard in back window
Bug guard (this is some kind of clear rectifier that is part of the front windshield and prevents cyber attacks)
Modifications that allow the vehicles to exceed 180 km/h (doesn’t list what that entails though)
They have a bunch more info on that page about the patrol cars here, but I could probably spend all day on it and not really get anywhere, a lot of that Japanese and Kanji is beyond my level.
In regards to the fast ones (the R34 GTRs and NSXs) I’m not sure, but I’d imagine most of the same stuff applies as the majority of the modifications seem pretty normal. And, just so this post isn’t text only, here are two nice patrol cars!
Hope I helped a little!