I did the same thing on my 03 road king, but I put on a aftermarket speedo and tach. the unit that plugs into the speedo also does the tach, and pin #6 receives the info for the tach. there should be 2 empty pins on the plug, it should be a 12 pin plug. #6 and # 3 are empty, my instructions said to make sure you leave pin # 3 empty, cant remember why, just it was specific. Now if pin # 6 is empty you need to run a wire to the nacelle, in there is a pink wire that is tied off somewhere, mine was off to the left side(as your sitting on it). This pink wire is there on all road kings, depending on model if they use it our not, standard on police models I'm told, so if your dash does not have a tach they just tie it off in the nacelle. Once you find it hook your #6 pin wire to this pink wire and your tach should work.
Now you can get the pins from Harley, get a wire pick, any part store has them and in your manual it tells you how to take the plug apart, add the pin to the wire and install it in the plug. It is really simple once you get it apart and see how it goes back together.
If I remember right the plug goes
1 12
2 11
3 10
4 9
5 8
6 7
check your manual for sure. Hope you have a Clymer repair manual it helps.
Hope this helps, if you have any one else see something i missed please chime in. Mark