GPS is an exception to the windshield law in CA. They added it I think this year or last year. I remember reading it on AAA magazine. You could have a GPS at the lower corner of the windshield. So as long as the police can't tell if your device is a radar detector or GPS, I don't think you would get pulled over.
You don't need radar detector.. just use WAZE app. People like to report cops locations in SoCal. I have yet to be surprised by any cops ever since I used it.