ON/OFF pulses to rear brake light of sportbike
As someone else has said, this would be pretty easy to do with a microcontroller, so I thought I'd try a discrete logic approach for fun.

I'm using a CD4022 octal counter as a state machine.

I'm also including a clock circuit, which provides a free-running 1/4 second clock pulse (or whatever you decide the timing should be) going into the CLK input of the counter. It uses a 555 timer configured in an astable mode.  Here's a calculator which I used to figure out the required component values.



Here's how it works.  When the brake is off, BRAKE will be at ground and the N-channel MOSFET Q3 will be off.  So the pullup resistor R4 will keep the counter reset line RES high.  Q0 will be high, which is not connected to anything.  The other two MOSFETs will be off, so the BRAKE_LED lead will not have +12v on it.

When the the brake is on, and BRAKE line is at +12v, Q3 will be turned on, bringing the RES line low and allow the clock input to increment the counter.  The Q7 output of the counter is also low, which is fed into the CLR line (called !CLOCK_ENABLE on some datasheets for the 4022).  Sorry about the confusion between the Q outputs on the counter and the MOSFET designations; can't really do anything about that.

The counter counts to 1, and Q1 output goes high.  This turns on the N-channel MOSFET Q2, which then turns on the P-channel MOSFET Q1, which causes +12v to be applied to the BRAKE_LED line.  after 1/4 of a second, the counter advances, Q1 output goes low and the Q2 output goes high.  Now MOSFET Q1 is no longer on, so the BRAKE_LED line goes back off.

This repeats for two more cycles (Q3/Q4 and Q5/Q6), generating three on/off pulses altogether.  Finally the counter advances to Q7, which stops the counter from advancing further because the CLR/!CLOCK_ENABLE line is now asserted.  Meanwhile Q7 high also keeps the Q2 MOSFET on through the diode, which is there so the earlier assertions of the Q2 gate lead don't stop the counter by feeding back into Q7.

When the brake is released, BRAKE goes back to ground, turning off the Q3 MOSFET and  resetting the counter.  All FETs are once again off, and the BRAKE_LED line is inactive.

The BRAKE lead doesn't need to be debounced, since if it bounces back from +12v to ground a few times 10's of milliseconds apart, it will just keep resetting the counter and starting over which will not be visible since the counter only increments every 1/4 second.

The parts are all available in through-hole packages from Digi-Key.  VN10LP is an N-channel MOSFET in a TO-92 package.  ZVP2106A is a P-channel FET also in a TO-92 package.  The two ICs, LM555N and CD4022, are in DIP packages and can both be operated directly off of 12v so there is no need for a step-down regulator.  All caps should be rated at 25v or higher.  Resistors can be either 1/4 or 1/8 watt; none of them carry any appreciable current.