Maybe this will help others.
Just installed a SiriusXM unit with my new Kenwood DDX374BT. I did not want to put holes through the roof. I placed the SiriusXM antenna in the roof air vent. It is under both the air vent and the MaxxAir cover.
It is held in place with the magnet antenna sitting on top of the metal brace of the air vent.
I used wall cable channel to hide the wiring.
The reception is fine from under these plastic covers.
Paul