momentary latching circuit?

You can use something like this- for a half to one second pulse use a smaller cap like 1 uF, or a smaller resistance than the 1M. Just change the values to change the amount of time the relay stays on.It'll trigger when you close your switch to 12V, and reset when you bring the input back down to ground. If you just use an on/off switch you'll need to put a resistor on the input to ground, to discharge the cap after you open the switch, otherwise it won't reset for the next pulse.

https://www.the12volt.com/relays/page5.asp

Constant to Momentary Output

The capacitor allows the coil of the relay to be energized until the capacitor stores a charge, thus de-energizing the coil. The resistor bleeds off the charge of the capacitor when positive voltage is removed from the other side of the coil. You can increase the output time by simply changing the value of the capacitor. This one will give you about a 1/2 second output. 

John DeRosa (Hotwaterwizard)

Stockton California

When in doubt, try it out !