Is the wire harness on a 1978 Chevy truck same as an 1988 truck?

No, the wire harnesses for a 1978 and 1988 Chevy trucks are not the same. There were significant changes in technology, electrical systems, and even body styles between those years. The 1988 model likely has far more complex wiring to accommodate things like fuel injection, more sophisticated emissions controls, and possibly other electronic features not present in the 1978 truck.