Can an auto insurance carrier designate their med payments coverage secondary to any other health insurance in CA as a result of an auto loss?

This is sticky, not sure what you mean by 'designate' secondary. Assume you are saying your health insurance has to pay first and med pay picks up what's left? Yes, in some states that is the practice/allowed. Some states allow ''double dipping'' meaning you can collect from both. Contact the Dept. of Insurance for your state, (should be an 800 number) they will be able to tell you. Also look in your policy it will also tell you for sure.