I don't think the OP is indicating he is having any trouble receiving the FM stations, simply there is low audio.
That would rule out an antenna issue - even if it was you should be able to hear loud FM 'rushing' noise if no station is being tuned.
Since the monsoon amp only has a single input from the radio for AM/FM/XM, anything beyond the radio is probably not at fault.
Sounds like something internal to the radio
Re-reading the original post a few times, I agree, it has to be inside the head unit, either the FM tuner output stage, or downstream one step at the input (source) selector.
But dxG6's idea struck me as having merit. There are some antenna path failures that can kill some part of the spectrum - FM, in this case - while the AM still passes thru.
That said, if the FM can be heard, albeit very softly, then the tuner is working. But there's a gain stage in the FM tuner section or just downstream of it that's not doing its job.
At that point, I'm not sure what I'd do. Getting the head unit repaired can be costly. Replacing it with a junk yard unit is a question mark for me - I don't know how different the Monsoon head units are than any other factory units.
Going to an aftermarket head unit has the upside of getting some of the latest features (blue tooth, screen for backup camera, HD radio, etc) but the XM may be lost or require a new tuner suitable for the new head unit.
If a reasonably priced repair can be made on the head unit, that strikes me as the first choice. After that, I'd probably start looking at an aftermarket head unit, and possibly a new amp and XM tuner to go with. As I recall, one downside to this, besides buying all that gear, is that there are some driver features in the radio that you might lose, or would need an external replacement adapter, as I recall.
I forgot about steering wheel controls. Not sure what's entailed with an aftermarket head unit.
[edit2] You can ignore some of my post above. According to this Crutchfield article
"The standard G6 system consisted of an in-dash AM/FM/CD single-disc player with six speakers. XM satellite radio and an MP3 player were added to the package later on. The premium system used the same radio
with an 8-speaker Monsoon system."
So, this indicates any comparable GM radio should work. A cheap unit from the junk yard may be all that's needed. My only concern is making sure it's not locked. HTH.