Okay - thank you everyone here and on the mistunation group, I got it installed and working. Long story short, I validated the harness pinout by checking each pin for the always hot battery wire, then verified the front pos and neg speaker wires. From there I found there was a factory ground on the Montero 9 pin harness that was mating to an unused pin on the radio, I need to add a ground from the radio chassis to the metal mounting brackets of the Montero and then everything worked.

For the sake of documentation, the factory gen 1 mitsubishi radio harness (at least for '91) is as follows:

5 Pin connector (viewing the mating holes straight on, with clip on top)
Top left pin: Black and blue wire = left rear speaker neg
Bottom left pin: Black and red wire = right rear speaker neg
Top right pin: Black and blue wire = left front speaker neg
Bottom Right pin: Black and red wire = right front speaker neg
Middle bottom pin: blank or blue power antenna wire

9 Pin connector (viewing the mating holes straight on, with clip on top)
Top row furthest left: white and red wire = right front speaker pos
Top row 2nd from left: Yellow and red wire = right rear speaker pos
top row 2nd from right : Black and yellow wire = dimmer
top row furthest right: Green and white wire = Illumination (tied to taillight)
Bottom row furthest left: BLACK GROUND (this was not used in my Mitsu Stealth radio as well as the aftermarket cd player that I removed)
Bottom row 2nd from left: white and blue wire = left front speaker pos
Bottom row middle: yellow and blue wire = left rear speaker pos
bottom row 2nd from right: red and white wire = Battery lead - always hot
Bottom row furthest right: Blue and white wire = ignition lead - hot @ "ACC"