If you live and reside in a neighboring state but work in CA, so long as you are not using that vehicle as part of your employment (using it for business purposes), you are exempt from registering. I know several who live in AZ and work across the border in California and were told by CA DMV they are exempt so long as they are not making staying in CA a normal part of their life.
If you work in CA and actual reside in CA, or you establish CA as a place of residing (long term RV is included), or you get mail at a CA address, you must get a drivers license and vehicle registration as soon as you start working in the state. I think the key is you are not living or spending nights in CA while you work.