Dudes, the FM thingy has been known to be a problem since forever. Usually there is a chip both inside the HU650 and HU850 that goes bad(implicated), it is the FM receiver/DSP chip SAA7709. It is implicated to go bad leaving you without FM radio OR crackling/popping sound that comes and goes.(does not affect CD, AM whatever else)
However it is indeed possible for the antennas to go bad too or have a loose connection. There are two for multipath diversity. One in rear bumper and one around rear windshield. Also, either both or one of them has internal power circuitry to boost the signal(power comes from HU unit), that can also fail.
So you have THREE possible sources for the no FM situation. For instance I also have no FM radio but it comes back when the car/radio heats up sufficiently and goes away if I use the AC and the car is cooled off. When it starts to cut out/crackle I know its either coming back or going away.
Mind you I use a bluetooth fm dongle. But I've also recently noticed that it also cuts out if I am in a region with shoddy FM reception(while still using my fm dongle)
It is a really complex problem that no one has found a cure-all fix. Some replace the antenna and have it working, other guys replace the SAA DSP chip and also get it working, other people have no success with either and end up replacing the entire unit which works for a year or so before the unit exhibits the same issues.
But don't take my word for it
https://www.matthewsvolvosite.com/fo...ic.php?t=97129
https://www.volvoforums.org.uk/showthread.php?t=275636
https://www.elektroda.pl/rtvforum/topic3718989.html (non-english but you can use google translate)