Build an astable 555 circuit that turns the output on for 10 seconds then off for 10 seconds. Google it or see
http://www.ohmslawcalculator.com/555-astable-calculator for circuit and capacitor and resistor values.
Connect the 555 output to the gates of two mosfets, a P-channel and an N-channel. See
**broken link removed** and search for "Complementary MOSFET Motor Controller" (last topic on the page), although as noted there could be a problem at the moment of switching if both mosfets conduct at the same time.