you wont find components of a valid defense in the "traffic laws", you need to check the MUTCD requirements and see if the stop sign's appearance, location, etc conform to the federal requirements of a legally enforceable stop sign.
just because a sign says stop doesnt always mean it's a stop sign.
here is a start for you taken from the MUTCD, 2000. be sure to see which version of the mutcd california is using.
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Section 1A.04 Placement and Operation of Traffic Control Devices
Guidance:
Placement of a traffic control device should be within the road user’s view so that
maximum visual acuity is provided. To aid in conveying the proper meaning, the traffic
control device should be appropriately positioned with respect to the location, object, or
situation to which it applies. The location and legibility of the traffic control device
should be such that a road user has adequate time to make the proper response in both
day and night conditions.
Traffic control devices should be placed and operated in a uniform and consistent
manner.
Unnecessary traffic control devices should be removed. The fact that a device is in
good physical condition should not be a basis for deferring needed removal or change.