I have the JRC 1000 which is the smaller sister of the 1500. Mine fogged up on numerous occasions almost from new. It is on my steering console, out in the wet, though covered by its own sun cover and the tonneau canvas console cover when we are not under way.
I investigated and it was a known problem to the extent that their UK agent, Marconi Marine, took it into their workshop in 2003 and reworked it FOC even though it was out of warranty.
Whether this would still be the case I don't know but you should try approaching the US main agent. Even if he does not do repairs, he might give advice on what needs to be done and so you might open the case and fix it.
BTW I also have Raymarine/Autohelm ST50 instruments and these are designed to leak too. This occurred one very wet day and I saw water swilling around inside my log LCD display. We opened it up, washed it out in fresh water, hung it out to dry and all was well. More recently my depth sounder failed and, when opened there was a 'high tide mark' half an inch up from the bottom of the printed circuit. I bought another unit from a jumble and it was just the same.
So I now cover the four fixing screws on the rear of the case with black tape and then put a band of tape right round the red rubber seal.
Similar treatment might fix your radar!
