"When the AI knows how to turn this on.

By default, the official AoE2:DE AI is not able to train Cobra Cars, even if you make them available in a building such as a Stable. However, by using the Advance Genie Editor, you can trick the AI intro treating the Cobra Car as another unit, such as a Knight-line unit (as seen in this video).

My thread on how to do this:
https://forums.ageofempires.com/t/getting-the-ai-to-recruit-custom-units/206409"