It is possible that the windows are controlled via CAN or LIN bus, which means you have to send a sequence of data to them. Pretty easy for a microcontroller - if you know which data to send. Since window open/close function is a security issue, the data might also be not that trivial, and contain some kind of encryption, CRC, or other features. And... the controller of the car might notice that there's something going on, and at least throw an error.
But it's also possible that an i20 has just simple motors at the windows.
First of all, your driver windows lever has two contacts for each position. Touched with little pressure moves the window until you release the button, a bit more pressure makes it move until the end. I doubt that the other levers have the necessary contacts for that.
The windows will just have a motor connected to 12V to move in one direction, and connected in reverse polarity to move in the other. What you are looking for is a H-bridge, which allows exactly that. It can be made of four relays, but there are fully integrated ICs for this. Have a look at Infineon Trillic series. These ICs allow easy direction switching, easy PWM for power control, and - that's important - overcurrent protection. Overcurrent protection allows to detect when the window is fully opened/closed and so when to switch off the motor. I will also detect when something like an arm is in the way. And they can power a motor from 12V, while they can be controlled by the typical 5V or 3.3V from microcontrollers.
I don't know if such ICs are available ready for use in the adurino world, but this should be no problem for someone who can solder.