What is the best way to get an inexpensive satellite radio module for my 2004 A-8? The last time I checked the dealer wanted $500 for one.

 

Consider buying one from eBay Motors. My A8L came with an XM module and I already had a Sirius account (plus I preferred Sirius' quality, programming, etc) so I researched (read contacted the local dealer for a quote and the P/N was on the written quote) which module I needed for my car and did a Google search. This lead me to eBay Motors. I bought my Sirius module for about $220 -- shipped. I sold my XM one for about the same.
It's all moot now since the merge so I think either module will work for your application.

Good luck!

P.S. I have no affiliation, nor do I profit either directly nor indirectly by mentioning eBay or eBay Motors.