I need a gauge cluster with the one plug in the back cant find one anywhere

 

??
Are you talking about the ralley pack with the tack, oil and water temp gauges built in? If so you need to look long and hard for them because the 78-80 gauge sets are getting rare. They computers yet so they are s bit different than the 81-88 units.
Also it is not a plug and play with the granny speedo either. I did the conversion once and all bu tone wire needed to be relocated in the harness to make it work.

Oh and the one wire was a ground.

 

not ralley pack just stock cutlass supreme and my wire harness only has 1 plug on it and all the gauge clusters i found have had 2

 

You need to find one pre-computer. They added computers mid year 1980 so that makes finding replacement parts that much harder.

 

ya i think im just gonna build a custom dash because i need a speedometer bad