Pick up a terminal (P/N 74196-04) from Dealer since it fits into the speedo Connector perfectly in the number 3 opening in the plug on the back of the speedometer or you can also use the #3 position in the plug at the ECM . There seems to be a problem with the speedo plug operating the tach on some bikes.
The Red Pin needs to come out; and partial disassembly of the Connector will allow this.
The Service Manual will give info for various connectors.
There's an empty P&A Ignition Circuit in the Fuse block that's perfect for powering the tach.
Use wire terminal (P/N 72309-04) or(P/N 72527-01.) plug it into the bottom of fuse block, insert a 2a mini fuse, and once the black wire is terminated to ground everything should work perfectly.

You can also use the #3 open terminal on the ECM plug as seen below for the tach connection on 06 bikes, some have found that the speedo connection on the #3 position does not power the tach correctly.

Here is a very good pictorial on the hookup.
Tachometer - Thumbnail View (http://www.shareaproject.com/pages/projectThumbs,p,173,00.html)


http://i149.photobucket.com/albums/s45/bills985/Tachhookup1.jpg

http://i149.photobucket.com/albums/s45/bills985/Tachhookup2.jpg

This is pertty much the same view as on the Sportsters, has anyone tried it on the 2008's ?