Radio antenna issue
 

I own a '91 rx7 convertible with a power antenna. The motor works but the antenna won't extend. I bought a replacement antenna but can't budge the nut to remove the antenna. A mazda dealer service department in my area couldn't help. Is there a tool out there to do this?
I know technology has made the need for radio antennas pretty nil, but since I have a replacement I'd like to replace it.
any help is appreciated. Thanks.