We will covered how to get an ETC card very soon, which is significantly more difficult for foreigners (especially the military), but now let's look at getting the box installed.
First of all, go to an auto-mobile shop/garage and choose a box. ETC boxes come in various price brackets, though the added benefits of the more expensive options are limited (if you can even work out what they are), so I'd recommend just getting the cheapest.
There will be two prices on the box. The cheaper of the two is the unit price, the second the price including installation. Boxes start around 7,000 yen, and you should be able to find one including installation for just under double that.
When you take the unit to the counter, they'll ask if you want it installed.
You may have to wait for a bay in the garage area to be free, at which time the staff will say something like "The wait time for a bay is .... minutes, are you OK waiting that long?" If time is an issue you'd want to make a reservation beforehand.
For those needing help, I'd say something like this to make a reservation:
ETCを設置していただけたいですが、来週土曜日の予約をお願いできますか？After you've made an appointment (or been pleasantly surprised), you will have to fill out your name and address on a form, and give them your shaken certificate to photocopy. Later, or next, you will drive your car to the fitting area and negotiate where you would like the machine to be installed (the antenna is always behind the rear-view mirror). Literally almost anywhere is fine. Mine is just under the steering wheel. They remove part of your dash to fit the box, so you might not want to watch if you're sensitive about your car!
(ETC o setchi shite itadaketaidesuga, raishū doyōbi no yoyaku o onegai dekimasu ka?)
- the underlines words being the day you want to make the reservation.
At the store, if they were explaining that you need a reservation they'd probably say something like,
(Yoyaku wa hitsuyōdesuga. . .)
"You need to make a reservation"
(Mata ashita no gogo ni kitararemasu ka?)
"Can you come back tomorrow afternoon?"
The fitting process takes 30-45 minutes. Then you're all set.
- If you move prefecture and change your licence plate, you will need to get your ETC paperwork updated.
- There are some "one-piece" boxes which don't need to be fitted (they sit on a holder on the dash and have the antenna built-in), although I've never seen one.