your building a all-indash setup right?
you should be able to use resistors to drop how much voltage gets to the fan. just connect in series with the red wire, or in series with the black wire. if you have a radioshack nearby, they have a assorted 1/4w, or 1/2w resistor kit that would be perfect for this.
with the interferance issue, i assume your using the pc power supply to power the fans. i wonder if you connect the fans to their own buck- voltage regulator to separate them from the pc components. the second idea is to use RF chokes, or different vga cabling for your screen..
but if you have a all-indash setup, i wonder if the interference is due to the close proximity of the fan/screen. in this case, you might need to look into shielding the pc components.. an easy way to test for this would be to connect the fans to a external power supply, and see if you still have the issues..