filter cap

I am mounting a 120 cm 12 Vdc cooling fan to my RM ITALY KL 60 amp for 12Vdc mobile use. Both units will be supplied in parallel by a dash-mounted rocker switch. I want to make sure the fan doesn't induce a buzz into the xmit of the amp. Amp frequency range is 26-28 Mhz. How do I figure out what...