If you are interested in doing your own repairs, the Bentley manual does a good job of walking you through the procedures.
And you have my sympathy if you’re facing a bungled car stereo installation! Some people should be banned from ever picking up a tool.
Mini Mania, ECS Tuning and Pelican Parts are suppliers that are located there in Cali, their sites have forums where you can no doubt find someone local to answer your questions or fix it for you.
Or if you’re interested in trying yourself, there’s a lot of people here who can talk you through it.