It was a while ago with I added Sirius to my E39.
After reading what n_sievers posted, I do vaguely recalling something with the BM53 radios on the 2003 era vehicles. Somewhere along the way, a BM54 radio was released as I recall.
I think the BM54 radio fully supports the Sirius modules or some sort of additional input.
This might be a good resource -
BMW Bluetooth Retrofit Information
There may be superseded Sirius module part numbers as well. Some of the newer units only have a single antenna input compared to the older units which may have had dual antenna inputs.
Search for a used module with Lifetime Sirius, I found one for my E46 for like $225, needless to say the module has paid for itself!!
I have Lifetime Sirius in my E70, so I will likely swap modules when I need sell the E70.