Help to ID a Diode? I think
Knowing exactly how this component was part of the original circuit, would help confirm/deny some hypotheses - i.e. you reverse-engineer that part of the circuit.


  I’m not sure if it a Zener, Schottky, or rectifier.


I suspect none of them.

The marking DEM as seen on the photo, is used by some manufacturers for a 15V standoff bi-directional TVS diode e.g. Littelfuse part number SMDJ15CA (datasheet on the range, see page 2) although the component in the photo seems to be from a different manufacturer which I haven't identified. That hypothesis of it being a 15V TVS, would make sense for a 12V power input as you describe (with some voltage tolerance).

I suspect this component was directly across the input power rails. If you confirm that, it would strengthen the confidence in this identification.

TVS diodes are used to clamp voltage spikes. Devices don't require them to work normally. Since your camera doesn't work even after removing the TVS, then the TVS isn't the cause of your problem. Your camera's fault is elsewhere, unfortunately.