Starting the Unimog activates voltage on the #61 terminals.
As engine speed increases, voltage on the #61 terminal increases.
The Red GEN lamp brightness is proportional to the difference between the #61 terminal and the B+ terminal voltages.
The B+ terminal is at the truck system voltage level, 26-28 VDC anytime the key is on and the DC switch is on.

When voltage on the #61 terminal is high enough, the fine wire relay in the controller closes, supplying voltage to the Radio box via contact closure. If the current goes too high, the heavy coil reverses polarization and drops out the relay.

If you have any questions or comments,

Thanks to Bob Ragain for the info and diagram. (More to come later...)