B&J ...
I have a 2004 Journey ... and XM receiver ...
I found that my antenna picked up the signal just fine under the fiberglass roof so I installed my antenna as follows.
I removed the headliner in the top of the cabinet above the driver (two or three screws) ... there is a square metal beam that supports the roof and the front TV ... I fastened the antenna to the top of that metal beam ... I drilled a hole in the cabinet so that I could get the wire behind the left window post covering ... and behind the dash ...
My antenna works just fine except when I am going NNW there is one little direction where it cuts out (I think it must be in the shadow of the air horn) ... when that happens ... I sing ...
The point of this is that you may be able to install your antenna without putting it on top of your roof.