Buy a flashing LED... About $2 from anywhere (Mouser, Digi-Key, Radio Shack)
If you need to use the actual lamp, try this circuit
for syncronising LEDs. Replacing the "normal" LEDs with your indicator lamp, and change the 47 Ohm resistor to about (14V - 3V)/15mA = 800 Ohms.
A little more complicated is this 2 wire lamp flasher
for lamps under 10W.
Wanna get home brew? Use a 555 Timer Circuit
. The output is only good for ~200mA, so you will probably need a transistor (like in the flashing LED circuit) to boost the output current.
MAKE SURE YOUR CHARGER WARNING LIGHT FUNCTIONS WHEN THE CHARGER IS CONNECTED BUT NOT POWERED UP. That way you still get a "my car is plugged in" indication in the event of a power failure or popped circuit breaker!