I would agree with a bypass.
If you do not use serious transient suppression for the circuit, you will kill the 555, 4017, and the LED.
When you turn your headlamps off, when your windshield wiper motor starts and stops, fan motor, etc.. You can get upwards of 70v on the system.
This is called a LOAD DUMP. The inductance in the alternator is still full of operating current when a device is switched off. The larger the load, the larger the dump.
TVS to ground at input buss for each "blinker". (Use automotive rated TVSs)
Take a look at what a standard automotive diode is rated at:
https://www.electro-tech-online.com/custompdfs/2010/12/Littelfuse_TVS-Diode_SLD.pdf
50,000W !! 76amps. 1 pS
Automotive environments are some of the hardest for a little IC.
ALSO, reverse battery protection. IF you have your hazard lights on, and give someone a jump start, a reversed battery connection can spell the end for your circuit.
These are some of the reasons that they are illegal in many many places.
If you get pulled over, or you loose a circuit, the ability to "reintroduce" the old bulb with a flick of a switch would be nice.