First where did you test the brown wire and find it measured 0.5 volts when the ignition switch is in the "start" position?I checked the Red and Brown wires, as suggested, with the ignition at start and got .52 volts.
The brown wire connects the battery positive terminal to the ignition switch. It originates at the starter solenoids battery terminal passes through the 3 cavity connector on to the ignition switch terminal 39.
Have you tested the voltage at the battery terminals with the ignition switch in the start positions?
I assume if you changed starters the brown wire connection to the battery cable at starter solenoid is clean and tight.
I might be reading something wrong but I don't think the starter would spin but the Bendix not engage if the voltage is only 0.5 volts.