I had a rev counter fitted a few months ago. I had it done at an autoelectrician and got my dash tidied up at the same time (I posted some pics on here at the time).
My alternator didn't have the w terminal fitted so the autoelectrician had to do that at the same time.
Unfortunately, the tachometer was fairly expensive: it was the only one I could find in NZ that was the correct diameter for the spare place in my dash. It goes up to 4000rpm off the top of my head, which is plenty for a big old diesel.