If you are adding to an 03 then chances are the BM53 radio module does not support Sirius but you would have to confirm that by looking at the part number on the BM53. There used to be someone in CO that did the reflash to allow Sirius support on older BM53's but not sure if he is still around. Here is a list of the BM53 models #'s and feature support, you may get lucky and have a newer BM53 installed.

BM53 Radio Support with Part Numbers
65.12-6 904 213 No aux input, no Sirius, no MP3
65.12-6 904 214 No aux input, no Sirius, no MP3 (re-manufactured part number)
65.12-6 919 080 No aux input, no Sirius, no MP3
65.12-6 919 081 No aux input, no Sirius, no MP3 (re-manufactured part number)
65.12-6 922 512 No aux input, no Sirius, no MP3
65.12-6 922 513 No aux input, no Sirius, no MP3 (re-manufactured part number)
65.10-6 933 089 Aux input, no Sirius, no MP3
65.10-6 933 090 Aux input, no Sirius, no MP3 (re-manufactured part number)
65.12-6 927 910 Aux input, Sirius, no MP3
65.12-6 927 911 Aux input, Sirius, no MP3 (re-manufactured part number)
65.12-6 943 449 Aux input, Sirius, no MP3
65.12-6 943 450 Aux input, Sirius, no MP3 (re-manufactured part number)
65.12-6 964 398 Aux input, Sirius, no MP3
65.12-6 964 399 Aux input, Sirius, no MP3 (re-manufactured part number)
65.12-6 972 664 Aux input, Sirius, no MP3
65.12-6 972 665 Aux input, Sirius, no MP3 (re-manufactured part number)
65.12-6 976 961 Aux input, Sirius, MP3
65.12-6 976 962 Aux input, Sirius, MP3 (re-manufactured part number)
65.12-6 988 275 Aux input, Sirius, MP3
65.12-6 988 276 Aux input, Sirius, MP3 (re-manufactured part number)