said by Hank:I have a Subaru Forester with sun roof, although I probably have more metal surface area than your Mazda3 the antenna is mounted within 1 foot of the AM/FM antenna. I do not experience any interference or overload with this arrangement.
I used an Larson with NMO mount.
Good to hear. I just wasn't sure exactly how robust the receiver front-end in typical car radios are nor how much RF power would actually be picked up by the out of band FM broadcast antenna.
Maybe I'll go ahead and mount it on the roof about a foot away and see how it goes. Thinking of picking up a 5/8 wavelength 2m antenna with NMO mount. Won't be right away though, as it's in the -25 to -40 degrees celsius outside and I do not have a garage nor does anyone else I know. Don't want to mess around pulling off trim inside the car in the cold as there's a high risk of breaking something (plastic gets very brittle in such cold temperatures).
Thanks for the info/suggestions everyone!