It works!
So I took the cable with the service switch into the shop. They hooked up the PST2 and enabled it. The switch has to be in the service position. They told me the menu option was something like this:
MOST > Position 2 > CD Changer
I haven't messed with it much, but once the changer is enabled you can disconnect the cable with the switch. You should keep it in the car though. I put my head unit on CD 6 and track 2 and was able to control everything through my iPhone.
I still need to try Pandora radio, or another non-iPod audio app. It wasn't showing track info on the head unit. Just the cd and track selected which controls what mode the Gateway is in.
I'll update with anything else I find out as I continue to use it.