The legal restriction
California limits windshield-mounted devices to specific areas: a square in the lower corner on the driver's side, a larger square in the lower corner on the passenger side, and a strip along the top centre behind the mirror.
A dash cam tucked behind the mirror is the standard answer and is compliant.
The coating restriction
Many modern windshields have an infrared-reflective metallic-oxide coating. It rejects heat, and it also blocks radio signals — which means a toll transponder stuck to a coated windshield may simply not read.
Manufacturers solve this with a small uncoated "window" in the coating, usually just behind the mirror, sometimes marked with a dotted outline or a small icon. If your car has a coated windshield, that patch is the only place a transponder works.
A transponder that used to work and stopped after a windshield replacement is almost always this: it was moved off the uncoated patch.
The sensor restriction
The area directly behind the mirror is crowded on modern cars — the ADAS camera, the rain sensor, the humidity sensor and the light sensor all live there.
Do not block the camera's view or cover the rain sensor pad. Both are genuinely safety-relevant, and covering a rain sensor with adhesive film is a good way to make automatic wipers behave erratically forever.
Practical placement
Dash cam: high, immediately beside the mirror stalk, wire run along the headliner. Transponder: in the uncoated patch if there is one. Parking permits: lower passenger corner. Nothing across the driver's sweep.
Need this done rather than explained? There is no shop to drive to. We come to wherever your car already is — your driveway, your workplace car park, an apartment lot, or the side of the road it stopped on — anywhere in greater Sacramento, with no call-out fee and no mileage charge. Give us the year, make and model and you will have a firm price before the call ends.