Hello fellow arduinist! You're probably going to need a raspi to go along with the arduino, as I'm not quite sure how you'd get echo to communicate with arduino directly without a LOT of hassle. Basically you're going to need something that can get json data and do something with it. Trying to do that on an arduino alone, if even possible, would eat up that onboard memory. Even with a teensey.
I've played around with a raspi and echo, pretty neat but it was a lot of work for a simple project. Being said, that was just turning on an LED with echo. Coupled with arduino and a relay board, it should be perfectly doable to make a system to turn on/off the lights (and nearly anything else electrical, since you can have multiple keywords for echo).
Just remember - echo likes that internet connection. Make sure you factor that in, since echo won't respond to anything without one (except setup).
Here's a starting point for the Alexa & custom skills - https://developer.amazon.com/docs/cu...our-skill.html
Also remember that there are amazon units for your vehicle too. Never played around with anything other than a dot, so I'm not sure about internet connectivity with the vehicle alexa, but it may be more promising.