Hi, I have a 2009 Cayenne with Bose radio.
I usually listen to XM radio and during certain foresty sections of my commute XM radio reception cuts in and out. I can still hear FM stations fine when that happens.
It's not an issue with my other car during same section of the road.
Also, if I open the rear glass portion of the hatch, xm radio gets zero signal.

Does Cayenne have a different antenna for XM? What items do I need to check when I remove the rear spoiler?

 

My brother has his XM antenna installed on top of the dashboard, inside the car, under the windshield. When you drive under tree branches, you lose XM.

I added an external XM receiver to my '08 (the only available connection option was FM radio... tune the car radio and the XM tuner to the same FM frequency, worthless in the Bay Area... so many FM stations and so much RF noise close to the bay, forget it, you couldn't find a clear-enough FM frequency to receive XM). That "free" tuner from Sirius came with a small magnetic antenna that sticks to the roof so well, it doesn't even come off in car washes. With that on top of the roof right above the driver's door, there is no cutting out when driving through MOST forests. We have been through some thick, dark, redwood forests and the XM cuts out there, but there is a lot of signal blocking biomass, so not surprising.

 

FM reception is from radio waves that bounce around everywhere. XM is satellite radio so needs a clear view to the satellites in the sky, so no direct comparison between the two.

Anything that obstructs that view (even passing slowly under a bridge) breaks that connection. Tinted or privacy glass weakens the connection if the antenna is behind them. The best you can do is put the antenna on the outside of the car.