The figure below shows a project / schematic of an automatic rain sensing windshield wiper control circuit. Now days some modern vehicles use optical based rain sensors to control wiper system but if your car don't have this system then you can make one by yourself with the help of this easy circuit.
The circuit presented here is not using the optical technology and it is made with another trick. It is built around the famous NE555 timer IC that is working as a monostable multivibrator here. The circuit is using a handmade sensor to detect raindrops. The sensor can be made by attaching 2 wires of 0.5 inch each on 1 inch square plastic piece. You can also experiment with other handmade sensors and make the one which best suits your requirements.

When two wire in the hand made sensor will detect raindrops then the output of the NE555 IC at the output pin 3 will goes high and activates the relay for a set time period and after that time period is over the relay will switch off again. The activation time period of the relay is few seconds but it can be increased by increasing the value of the capacitor Cx.

After building the circuit connect the switch or wires of the wiper motor with the relay. Connect long wires with the sensor and set the sensor at the corner of the windshield glass or any other place where it will get direct rain. Also make sure to set the sensor in 90 degree direction or in the direction of your windshield glass so the raindrops will slip over from the plastic sheet this will save from the triggering of relay when rain stops.  

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Automatic Rain Sensing Windshield Wiper Control Circuit Diagram