I am trying to replace the genuine Subaru satellite radio module in my 2013 Subaru Legacy 26i Premium. It was installed by the dealer who sold me the car used. It has an antenna mounted on the windshield. (the car's manual confirms that where the factory antenna would be) I have tried everything I can think of to improve the reception, which drops out frequently. Remedies attempted included having Sirius refresh the signal many times, moving the position of the antenna (magnetic rooftop antennas are incompatible) and of course having another Subaru dealer diagnose the module (they can't find a problem, though we took a ride and they heard the dropouts) . This module part is discontinued. It is H621SAJ000. A supplier of Subaru parts says a new part H621SAJ100 supercedes it. Here's the question: Does anyone have any experience with this switching of parts? Is it truly compatible, and is it a real upgrade?   Or any other suggestions? thanks.

The AM/FM antenna is built into the rear window but the satellite antenna is the sharks tooth at the back of the roof.