A band expander won't give you better signal quality, it will allow you to access a wider band of frequencies than those used by Japanese radios (76MHz to 90MHz).
regardless of the band expander you wont get very good radio reception in our area because of the window mounted antenna. You might be able to get power antenna installed in the rear quarter panel.
You can get a booster that will increase your reception - Carsten posted information about it in another thread (probably in the In-Car entertainment thread, I expect).