I would look for a head unit that comes with a steering wheel mountable remote. That would be the easiest.
I had a vw rabbit that did not have steering wheel controls. But I believe some did, and the GTI did for sure. I looked into what it would take to retrofit and it was a lot of work.
You needed the different wheel, change the hub to one that can transmit the signals. (also had to make sure the air bag would work with both the hub, and wheel, or you might need a new bag). Then on the VW you had to program the car to let it know you had the wheel.
Then your steering controls would work. 🙂
Oh and if you wanted your steering wheel controls to work with your after market radio, you would need an extra ~$100 adapter (if it exists)