External Antenna for the Magellan 3225?

Does the 3225 do real-time traffic?

Not sure but I think the external antenna on the Maestro line is for the traffic reports and not for the basic GPS function.

What does the User's manual say?

The 3225 does not do real-time traffic.. and the User's manual states the jack is for an external GPS antenna.

Sounds like an Email/call to Magellan is in order.
I think there is a "contact us" link on their site somewhere.

I'm not using an external antenna, but since you're looking to expand your mounting options, I put my MM 3225 on a generic air vent mount next to driver door and find that it works fine there. Internal antenna is getting max bars on the display.

Are you having problems with the signal? I use my 3225 inside the cab of my RV with a over-hang and still get a full bar signal, including city driving and bad Florida rain days!!! And...I get full bars in my Townhouse (downstairs)..Maybe I'm just lucky.

Maestro 3225 is not doing real-time traffic.
For real time traffic you need a subscription and GPS unit / device which support that.

The user manual is ... well .. just for basic information quite a poor user guide.

I guess the external antenna is for special situation.

I thought the antenna was for FM purposes. I thought I read in the manual it was for FM, but not working/future implementation.

Yes.....but not FM as you are thinking of it.
The traffic service data is transmitted on a sub-carrier of local FM radio stations. So the first answer you got is, in fact, correct. Your model doesn't have that capability. The "future models" that have that feature actually exist now.