This might be literally everything.
OBD2 port power?
I myself am planning to add one, as the OBD2 port is under the cover which needs to be unscrewed (bravo FIAT), visible in the bottom right corner of the 2nd photo and has constant battery power. So if someone was using an OBD bluetooth / wifi interface for example and wanted to avoid draining the battery, this would be a good place to install one.
Starter battery bluetooth status monitor?
Fog lights? Lightbar? Any other external lighting (step, garage, underneath?)
Reversing camera?