Activate auto door lock in i20

Sure, What you are talking about is totally possible. In this one, like most things in life, you can achieve almost anything with the application of Money / Time / Energy in appropriate mix.

• You could certainly figure out which wire energizes the locking circuits. This would require a complete wiring and logic diagram to the circuits (to ensure you don't send a stray signal upstream to adversely affect something else in the system.)
• You could read the vehicle speed off the communications network. I don't know much about the Hyundai I20, but I presume it uses a modern computer fuel injection control system, with networks messages, including vehicle speed. I'd use a raspberry pi device to read the messages. I've not done that job yet, but I'm assuming the data feed protocol is available somewhere.
• You may have to add isolators / relays (transistors, darlington or mosfet) and perhaps diodes to protect the existing circuitry in the vehicle. I'm pretty sure Hyundai uses electric motors for their door locks and not solenoids.
• Although this would be a fun project and totally do-able, its never as easy you might think at first glance.

I can think of other projects that might be more fun (e.g. read vehicle speed, change WS2812 LED interior lighting colors to match vehicle speed...) something that doesn't require risking existing devices within the vehicle.

Where are you going with this? Is it not just as simple as getting in the habit of locking the door when you start your drive (to ensure kids in the back seat remain within the car?) . How much time/energy/money do you want to devote to this endeavor?

And that first task, obtain a complete wiring / logic diagram could stop you cold. Without that you won't really know what you may be risking with wiring modifications. Generally dealerships / service centers don't provide that level of detail (and certainly not for free).

What I'm telling you this project can require a whole lot of time. I'd like to see you pick a project that you can learn, have fun and complete, without becoming discouraged.

Apologies for being slightly negative. I don't want to discourage you. I look at myself. I got interested in working in cars as a hobby, and I ended up working at an automotive manufacturer as design engineer. I turned a hobby into a career.

But hey, this is just one person's opinion. Others may pipe up with a different view. Good luck with it.