I need to manauly control a servo

[Lizard]

If you want a flip switch instead of the dial on the servo tester: I think it should be possible to use the right resistors. The dial on a servo tester is basically just an adjustable resistor, if you measure the ohm values it gives in the desired positions, you can replace it by resistors with those values and a switch.

Or maybe get some cheap 3 channel radio from hobbyking, throw away the transmitter case, steering wheel etc., put it in a small box and you have a radio-controlled license plate.

If you have a lowrider, you could also add a gyro to make the license plate always stay level when you let it bounce ;)