Hello.. I bought a 73 Stang a few weeks ago.  Runs great and is in great shape -- one thing that does not work is the tachometer... Just doesn't read anything anytime.  I was hoping you could point me to the easiest way to troubleshoot what could be wrong?  Where should I start?

Thanks for any guidance,  

Check the wiring.  The four wires are two red and two black. The red wire marked "trigger" goes through the firewall and is connected to the coil wire that goes to the distributor. The second red wire is the "hot" lead and is wired into the ignition wire where it goes into the fuse box, so that when the ignition is ON, the tach is ON, too. The black ones are ground.  

And if all else fails and the tach is bad, the August 2008 mustang monthly magazine has a great article on reconditioning speedos and tachs.