How do I enable bumper cars on any map?

While you can addcond 82 to players to do it using cheats, the bumpercar model and sounds won't be loaded if you do that.

You really need a server plugin to do it correctly. As of right now, there are 3:

• Kartify by Dr. McKay
• Bumper Cars by Chdata
• Bumpa Cars! by Matt_

Of these 3, the first two were updated just today. I would consider holding off on all 3 for now until their authors iron out all the bugs (such as team switching and such).

Now, having said that, it's possible to make new maps that use bumper cars and at least one is already underway:

This uses either the addcond 82 command mentioned above in a trigger_multiple, or uses trigger_add_tf_player_condition brushes.