Unstable input to trigger LED after 10-15 seconds (12 V)
This circuit should do what you want:



It uses a 556 IC, which is a dual 555.  The first timer is configured as a missing pulse detector (monostable).  With the pot set at 15% of its value (45K of the 300K), the time output period is 15 seconds.  With the pot set at 90% of its value With the post set at 60% of its value (180K of the 300K, the timeout is 60 seconds.   Note: these values are approximate, mostly due to the tolerance of the capacitor.

The voltage divider made up of R5 and R6 cut the 12V input down to 5V.  When the input goes high, it starts the timer (monostable).  As additional pulses come through, they retrigger the timer.    As long as the input is 0, or the first timer is running, the \$\overline{\small{RESET}}\$ lead of the second timer is held at 0 keeping the LED from flashing.

When the first timer expires and the input is high, the second timer, configured an an astable, is pulled out of reset. It flashes the LED at a 1 Hz rate (500, ms on, 500 ms off).   (If you want to change this timing, you can use this 555 astable calculator.)

Because the output of the 556 typically limited to 10 mA or less, and you will probably want more than that for your LED, I added an inverter with an open drain to the output which can sink up to 40 mA.  To save parts, I used another gate in the same package for the other inverter, even though it doesn't need the extra output current.  Because it's open-drain though, I added the pull-up resistor to +5V.

If you don't want the LED to flash, you can discount the input of the inverter IC2A and connect it to the input of the NOR gate IC3A, and then replace the 556 with just a 555 (the pin numbers will of course change, but the names will be the same).

If this circuit were to be placed in a vehicle, like a car, it would be a very bad idea to connect it up to the 12V battery directly.  First of all, the 12V battery is nominally about 14V.  In addition transients can be nasty on a vehicle's 12V system, with voltages rising as high as 125v for 10 ms during a load dump.

This power supply circuit provides protection against negative voltages in addition to the positive spikes due to load dumps, noise, and jump starting, and regulates the 12V down to a filtered 5V:



No matter what circuit you end up choosing, if it's electrical system is such that transients like this could occur, then you should include a similar conditioning circuit.