Instrument cluster

Hello

I have a 73 el camino ss. I am having problems getting my gauges to work. I took out the old instrument cluster to put in a newer one while also replacing the busted speedo cable. now the speedo works but none of the other gauges do. they didnt work before either, except for the gas gauge. i noticed their was one large connection on the back of the cluster but i'm not 100% sure it is plugged in correctly since i cant see or feel if it's in. I really just want the gas gauge to work but it would be nice to have the volt (http://forums.superchevy.com/70/7403210/tech-talk/el-camino-instrument-cluster/index.html#) meter to work as well. So my question is:

How can I troubleshoot getting the gas gauge to work? how can I be sure the connection in the back is hooked up? obviously, the first thing i'm going to try is put gas in the tank, but i figured since the other gauges didnt work then that gas gauge wouldnt either. Also, there are lights throughout the instrument cluster that do not turn on. I'm new to this.