This appears to be a starter (positive) battery engine sub-harness.
 
Not sure exactly what it fits, but STAR parts says:
 
Fits 1984-1987 M body only
 
Please do your own research BEFORE purchasing as all sales are final.  If you want more pics, I'm happy to take it out of the package and take more pictures for you so you can compare it to the one in your car.    From what I can see it definitely has the RED (positive) terminal/connector for the battery, and goes to the starter.  This is definitely a starter harness.  What it fits (for sure) I can't say.