If you have the Metra SWC, you might as well get the Metra GMOS-04 (or GMOS-01, depending if you have Bose or not). I have the GMOS-04, and it worked really well for me. IIRC, there's a spot on the GMOS where the SWC adapter plugs right in - they even give you an extra SWC harness (which you already have) to use.
Wire the GMOS to your radio harness in the house, then take it out and plug it right in to the GM harness after removing the radio (there are two harnesses, as I recall - Metra supplies both plug ends. With that, you won't have to cut or splice one wire besides what you had to do to connect the radio harness to it.
No, I don't have the SWC adapter - yet. I may break down and get it when I finish up my install - other things have priority right now.