Car flasher circuit and grounding
Here's one idea.

Figure 1. Image source: EEV Blog.
This one uses D1 and C1 to maintain power to the timer circuit  while it is shorted out by the MOSFET.
Be aware of a couple of safety points:

You also need to ensure that on power-on that the lamp turns on instantly not, for example, after one half-cycle of the flasher. At 100 kph you're traveling almost 28 m/s and that half-second delay could be significant in avoiding a collision.
The thermal flashers are very clever as they also blink faster when a bulb blows or is disconnected. To do this electronically is a lot more complex. The circuit of Figure 1 won't!
There is no audible indicator.
You may have trouble with the dashboard indicator lamps if they rely on a ground connection rather than +12 V.

I recommend the 2-pin electro-mechanical solution. The linked article has some interesting points made by various contributors.