I have a Pioneer AVIC-7000NEX that I have installed in my vehicle since November, about two months ago I started to get a constant "Check Antenna" message when listening to Sirius.  Everything appeared okay, but I still decided to replace the SXV with another one, now two days after putting in the new SXV I'm getting the "Check Antenna" message again.  It worked fine for two days, has anyone else had this issue?

Yes, I have the same issue with the SXV300 and my Pioneer 4100NEX.  I messed with some cables going into the trunk to make sure they're not pinched and that helped a bunch.  The problems only seem to happen now when my car's been sitting in the Alabama heat for 4 or more hours straight.  It always works in the morning after pulling out of my garage.  I wonder if the 300s are bad units.  

And appears it has failed completely

Mine went out again today in the 97 degree heat of the day working on the car. I think it's a heat issue

Yeah mine seems to work more in the morning than once it's hot out.

I wonder if this is only on the sxv300, never have had this happen with the sxv200.

I agree that it may be the sxv300.  I had the 200 in other cars and did not have this problem. 

My girlfriends SXV300 went out this past week with the same error message while on a road trip from Virginia to Michigan.

I wonder if it has anything to do with humidity.  It's still blazing hot down here in Alabama but the humidity has been lower this week and my tuner has been working much better.  

So I replaced the antenna with a round antenna, not the square one and so far no problems.

It's been hot here in Pittsburgh, but not ridiculous. I did notice that the module was very hot when I removed the radio.

My tuner/antenna still working better lately as our humidity has been lower that normal.

Spent 12 hours in the car today, temps between 75-90 no issues since the antenna replacement