Is it a CDM-XXXX or a CDA-XXXX? Yes there are new antennas that are a lot smaller. You would need the new antenna and a splitter box 'cause the new antennas use only one plug instead of the two plugs on your unit. None of the antennas available will work "just as good" inside the car as outside the car, but if you are willing to get the antenna high enough up, not behind tinted glass or defroster lines and might not mind a litle drop out you could probably place one under the upper dash panel and get decent reception. We put them under dash panels on a fairly regular basis and haven't had too many complaints. I guess it's just a matter of not getting your expectations too high. Another option is to go w/the smaller ant., pull the shell of of the ant. and have a body shop paint it gloss black , or whatever color your car is(I shouldn't assume your car is black just cause it's a GN) and then re-assemble it. It makes the ant. blend in a lot better. Anyway let me know which model your head unit is and I'll see what I can do. Do you already have a CD changer hooked up to the unit now?
ADAM