Hi Jarda, welcome
How have you determined that the signal is digital?
It might be easier if it was analog because there are circuits you can buy premade that can interpret those resistances. You can then programme a function for each resistance. I'm not sure how you would do it if the signal is digital, but it is something worth looking into, because in future the steering wheel signals may all be digital.
Regarding the relays, I've noticed in many cars, if not all, that the headlights (or sometimes only the high beams) are controlled by a relay. When you turn the light switch, you might hear a click from your fuse box/relay box, and you'll also hear it when you turn the lights off. If you can work out how that circuit is connected, you may be able to replicate it for your steering wheel controls. I'm not sure how the digital signal would fit in here, though.