I just put an L98 into my L03 car and have a mystery plug. It branches off of the engine harness right by the ECM under the dash. One side is white, the other is gray. My question is where do I plug the white part of the plug into the car's chassis harness? Please help!!

 

It sounds like you are talkign about the IP connector. If so it is what hooks up all the power and goodies to the engine. Does it look like this??? It should actually almost be clear, if we are talking about the same part. If this is is. That connector should be there under the dash

 

It's one of those IP connectors, just not that one. It's smaller and in the same area. It's a 6 pin connector.