Tom,
I am not aware of a restriction on a GPS antenna requiring it be mounted any distance from the cabin speaker magnet, that is where mine is located. I presume you have a mechanical interference issue.
If you are considering installing an Aspen PFD, the RSM which contains a GPS and antenna as well as a magnetometer and looks like a GPS antenna. It can't be installed close to the speaker as it would throw off the magnetometer.
Regards,
John Collins
CFI,CFII,MEI
V35A N7083N
Charlotte, NC 28211
(704) 365-1161 Home
(704) 576-3561 Cell