For the shield itself, there isn’t much to break that wouldn’t be pretty obvious. The EEPROM and ATTiny would be the two major components.
So for the ATTiny, the most basic test would be to use the Acc line. Provide power to the 12v line, the ATTiny should have 3.3 volts on it’s power pin. Then momentarily provide power on the accessory line. The ATTiny should sense this and turn the Arduino on, and it should stay on for 60 seconds, then turn off. This time is configurable within AutoSense if you want longer or shorter.
Ohhhh, since I didn’t configure your shield, you’ll need to set some toggles in the settings!!
With everything connected, go to Settings/Arduino/Setup Shield/Shield Capabilities. Click the “I Understand” toggle. Then in the list that follows make sure to enable “HD Radio”, “Input/Output” and “Temperature Sensors”.
That will save the capabilities into the EEPROM and setup the auto-configuration.