It's a shame that the unit number is no longer written on your driver's door by the VIN label. I have heard that some CHP cars also had their number written on the front license plate holder, or under one of the rear seat cushions.
At one time, you could request from California DMV a registration history for your vehicle, which was discussed here:
http://www.specialservicemustang.net...California+DMV
Due to privacy concerns, the DMV may no longer provide that kind of detailed information. But you may be able to obtain a redacted version of that report. Ideally, the information may list the CHP as the first owner, and include its original license plate number (your car's unit number).
Knowing the number will only go so far in determining where your car worked. Perhaps you will find an in-service photo with your car's unit number. We have had some moderate success working with CHP Motortransport to get copies/originals of the CHP Assignment Cards. Our success rate wasn't great due to the amount of time that has elapsed, but we did obtain a decent amount of the cards. However, due to a policy change a couple of years ago we do not (currently) have the same access as before. That may one day change.