I have an 89 IROC with 115mph speedometer (mechanical). There is an electrical connection on the back that goes from the speedometer to small yellow box/connector that goes to the main electrical connector on the tach side of the instrument cluster. I was wondering if anyone knows what this is for?

 

That's the VSS buffer. There's a spinning bar and an IR R/T cluster that is part of the speedometer. The VSS buffer converts that signal into something the ECM can use. Basically it's the link between your speedometer and the computer.

 

Thanks