There are cables available that will go directly from an RDP148 (or any NAVnet 1 or xv2 system) to a DFF1. The most common length is 5 meters, part Number 000-159-705. If you need one shorter, a 1 meter is part number 000-159-704. These are available from an authorized FurunoUSA dealer.
I'm not sure if you are continuing a previous post, but I'm not sure what cable you have. If you have one with an RJ45 and a 6pin that is a straight, you can convert it to crossover by cutting and rewiring the RJ45 side (cut off the RJ45 and crimp on a new one) as follows:
6pin --- RJ45
Pin1 --- Pin3
Pin2 --- Pin6
Pin3 --- Pin1
Pin4 --- Pin2