It's funny you ask, I ordered a Metra wiring harness ( the one for a 2006 325i) and got sent an american international instead. I never heard of them but decided to give it a go and if it didnt work I would just get the Metra.
I was also able to get constant off the harness as well. I know there were posts stating that constant wasn't available either.
The only thing I needed to do was run accessory to the fuse box.