It's been a while, but I've added steering wheel controls to my car using after market steering wheel controls from the Lancer/Outlander ~2010 models. These fit the existing 2010 steering whee, it even has the cable clamps in the plastic back cover.
You also need a new clock spring though, since the cheap people at Mitsubishi installed one that only connects the horn. The clockspring only cost about 10 euros, so that wasn't too bad. I sacrificed a old floppy connector to transplant into the existing white connector under the steering wheel column. I sacrificed a JST connector for the other side of the clockspring to use inside the steering wheel. Still haven't connected up the illumination wire.
I then made and installed a arduino nano, wrote and adapted some software so that I can use the cruise control knobs with the after market JohnGold Goldcruise controls. These were about 8 years old now and the plastic was giving out. It works a bit more reliable this way.
I control this one using a basic Aliexpress 4 channel arduino relay board, but the schematics I have are for a solid state variant.
Translating the Audio button using the arduino works pretty well, and I use the MCP4151 digipot to drive the 12 year old pioneer radio.
The code and schematics for what I used is here. Photo's are not yet uploaded.