What is this empty plug behind the cluster?

[dafro42]

I bought a 95 240sx with a sr20 swap in it. For the past two weeks ive been trying to figure out why the speedo doesn't work. All of the other gauges work. I replaced the speed sensor and it still doesn't work. I pulled the cluster and saw a relatively large plug that doesn't go to anything. What does this plug do?
Modified by dafro42 at 9:54 AM 7/17/2008