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Under California state law, the only knives that are illegal to carry in your car are switchblades or butterfly knives with blades 2" or longer (California penal code 21510).
Under California state law, there is nothing illegal about carrying any manual-opening folder, of any length, in any part of your car, either out in the open, or out of sight.
Likewise, under California state law, there is nothing illegal about carrying any fixed-blade, of any size, even double-edged, anywhere in your car, either out in the open, or out of sight.
Under California state law, carrying a fixed-blade in a bag is perfectly legal, even if the knife is carried completely concealed in that bag.
The law here in California that says it's illegal to carry a concealed fixed-blade specifically refers to carrying the knife "upon the person" (CA pc 21310). And there is case law in California that specifically addresses this topic (People v. Pellecer) which specifically says that in order for a knife to be considered illegally concealed it needs to be carried upon ones body or within ones clothing. People v. Pellecer specifically says that carrying a fixed-blade in a backpack, or other parcel, is NOT concealed under the law, and is therefore completely legal.
Anyone who wants to can look up People V. Pellecer for themselves by doing a Google search.
As far as carrying knives in your car, it's possible that some counties/cities in California may have there own laws. It's always a good idea to check your local laws as well as state law.
If you say what county/city you are in, or planning to visit, someone here might be able to help you find an official source for those local knife laws.
Here is a link to California's official state website where all of it's laws can be found-
http://leginfo.legislature.ca.gov/faces/codes.xhtml