If my 2003 Silverado's factory radio had the option where the radio volume goes up as the speed/rpms go up, does that mean the vss is right in the factory harness and i can plug the gmos-04 directly from the d3 to the oem harness since metra added this option to the gmos-04?

I thought the same thing, but I found that it's not the case. I thought my GMOS-01 was defective, b/c it would not get the signal and pass it through. Someone here said Metra is full of **** and their module does NOT do this. If you search for "GMOS" with me as the author you'll find a couple threads by me with the ongoing discussion.

FWIW, I received my replacement GMOS unit from Crutchfield today and will swap the old one out this weekend and report back, but from what I'm told (by impalacustom, i think)...I'll have the same "no vss signal" issue afterwards.

How this translates to your '03 w/GMOS-04? IDK.

thank you for the info i just got my unit today and am trying to be sure i got my ducks in a row.

Yeah, I just got my replacement GMOS unit yesterday as well. Heading out now to swap them out to see if that helps, and then do the legwork in finding the VSS if necessary.