Latching Relay That Follows +12VDC Trigger and Uses No Power Unless Switching
You need a capacitor large enough to hold enough energy to reset the relay when the +12 disappears. Minimum pulse width is stated as 30ms. Assuming a 1V drop would be okay and allowing for say 45ms time, you'd need a capacitor C = \$\frac{t \cdot I}{\Delta V}\$ or 0.092A * 0.045s/1V = 4,000uF. So maybe 4700uF/16V. 

Then you need a control circuit to detect the power loss and pulse the set and reset coils. It may be possible to do an adequate job in your case with a few passive parts and a 2N6028 but if you need reliable operation in more pathological situations such as slow brown-out of the 12V line you might want to consider more sophisticated circuitry or even a microcontroller.