You did not mention what kind of issues you are now having with the GPS.
That it worked fine for 3 years and is now acting up is interesting. Why not try a temporary non mounted location 6 feet away and see if it solves the problem. I doubt it will which should tell you something. My Lowrance is mounted under the cabin roof forward on the cabin and the radar is higher up on an arch at the back of the cabin. I would think I have about five or six feet of separation from the radar dome. I have found I have a lot less problems with in cabin GPS's that have a separate external antenna. You get much better satellite lock on and usually less positional error. My external remote GPS antenna is on the roof and also about 5 feet from the radar dome and is lower so hopefully not in the radar beam. Do you notice any difference when your radar is off, on standby or on, in terms of the operation of the GPS. If the problems occurs when the radar is on or on standby but not when it is off, that would suggest to me that it may be related to the radar. If the radar is just above the GPS and you have an internal GPS antenna built into the GPS the radar dome may be physically blocking some satellite reception but that would not explain why it work just fine for three years. Sounds like you have some testing to do. It would not surprise me that your current GPS problem turns out to be unrelated to the radar. In less of course you just installed the Radar right on top of the GPS and that is when your issues started.