Jetfour,
As painful as it might be, I think to get around this problem you will need to buy a GPS external antenna. I've seen them for around 40 pounds STG. This solution (not cheep I know) means that the antenna, which is picking up the rubbish from the radio, will now have the chance of being outside and away from the offending radio, plus it will increase the signal strength of the GPS transmissions as the antenna has a built in amplifier, hopefully tuned to the GPS freq. and not broadband as that would spell trouble all over again.
Another option I might consider if possible and that is to exchange the GPS/radio for a different one, a different manufacturer perhaps, if that is an option for you of course.
Good luck !
LL