Let me start by explaining that in order to search your vehicle, police need to have your permission, or probable cause that a crime occured and that the evidence would be in your car, or a search warrant, or an individual inside your vehicle that is on probation. If you are on probation and the officer knows this, he may search your vehicle. There are many other exceptions to your right to be free from unreasonable searches however the bigger issue for you may be to simply recognize that if what you have said is true, you are "on the radar" meaning they believe that by stoping you and searching you they will find something they are looking for.
In response, you should document these stops, including the names of the officers involved and any statements made. Oxnard Police Department records all interactions with the public. If you feel you are being harassed, you have a right to request that OPD command staff review these recordings for mistreatment.
Whatever you do, this would be a good time to make sure that you do not break any laws or allow anyone in your vehicle that might be carrying anything illegal.
Good luck with this.