* Year-to-Year Changes: Even within the same model year, there can be significant changes in wiring harnesses. From 1990 to 1992, Dodge likely made alterations to the electrical system, including updated components and potentially different connector types.
* Engine Differences (even if both are V6 or V8): While you mention both having a V6 and V8, the specific engine's computer controls, sensors, and wiring are vastly different. A harness for a 3.9L V6 will not be compatible with a 5.2L V8. Even if both were V6s, subtle differences in the engine's components would cause incompatibility.
* Options and Features: The wiring harness also includes circuits for various options and features (power windows, power locks, air conditioning, etc.). Different options packages between the 1990 and 1992 trucks would result in incompatible wiring.
In short, attempting to use a 1992 harness on a 1990 truck would be a massive undertaking, and likely unsuccessful. You'd need the correct wiring harness for the specific year, engine, and options of your 1990 Dakota.
