I tried the MP3 FF & FR on a D-NE326CK & it works. This player is the same
as the 320, but includes a car kit. It does it quite well, the sound does
not get blanked out, but it sounds like skips. It's not like a normal CD-DA
where it is just increased speed, the MP3s kind of skip ahead with audio
though.
It is probably the same on the D-NF420 also, I can try that one later if you
want me to. The NE326 was easier to get to right now, so I used that one.
BTW-I love the NF420, great player for the price!