Welcome to the wonderful world of Subaru stereo entertainment. They're just not known to have good audio/nav/interface equipment.
I get poor reception in certain areas in both my XV and Tribeca. Funny thing is my 03 WRX never had FM reception issues. oh well...
I'd check out Crutchfield.com for an FM signal booster. I think I've seen a device that you put inside your windshield behind the mirror to gain better signal. I'd go that route before I modified the external antenna.
If you've come to help me, you're wasting your time.