Have you looked at capacitive sensors ( a, b )?
If you want the sensor to detect presence/absence of the car, even when covered with water, cap-sense detectors are not for you.
If you merely want the sensor to survive power washing, and then, after the water has run off, to resume working, then perhaps a cap-sense detector could work for you.
Mark Lee. "Build A Touch-Sensor Solution For Wet Environments". Electronic Design 2008. a for tips on building a cap-sense detector with a 3-way "presence", "absence", and "covered with water so I can't tell if there is presence or absence" output).
detect objects within 10-50cm
In theory, this should be easy with large enough cap-sense plates.
It may be exposed to sunlight.
Capacitive sensors are immune to sunlight.
Cheap. Really cheap.
Capacitive touch sensors can be surprisingly low-cost.
As you can see from
"Two-element capacitive touch sensor" ( c ),
a few spare I/O pins on a microprocessor,
a few resistors,
and some plates of metal,
are all that are required to make a capacitive touch sensor.
Other articles show how the capacitive touch sensor continues to work even when completely sealed inside a water-tight plastic case.