Yeah, sounds right. I fitted a Pioneer unit to my old SLK, and spent ages trying to find out which wire was the speed pulse. In the end, just plugging in the ISO plug did the trick. A green/grey wire rings a bell, but it was a couple of years ago, so I could be wrong. I didn't bother with the reversing feed.
Mine was a Pioneer all-in-one head unit, so I didn't really have any cables to run back, other than for the CD changer, which was mounted to the base of the boot, and the cable running down the side, along the door sill, then up behind the glove box and finally to the back of the head unit.
BTW, the sat nav will work fine without the speed pulse until you try it in a built up area (i.e. when you need it most!), at which point the close road intersections and fragmented gps signal will cause all sorts of confusion!