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Hi Scott,
Unfortunately I am not familiar with the Thunderbox. Do you perhaps have a link to a manual?
‘I see the green light start to fade and then go off’, do you mean the green LED of the K6 or of the Thunderbox?
When you turn on the ignition, is there voltage on the yellow wire? Can you measure that?
If you hold the yellow wire against the connection of the red wire, does the K6 turn on, so you are bridging the Thunderbox for a moment.