I guess we are talking for white leds only? Yes, in one straight ahead view they were great but what happens from an angle viewing? I feel that modern cars with leds use different lenses than in the old cars, perhaps with more light diffusion. More over, modern cars have a lot better stabilization voltage or current systems for power supply to the leds.You might need to set an array of leds in series which supplied from a 12V system to
handle the 12V, since each one led needs 1.25V ( or around that). Or, use the well known limiting resistor. In an old charging system which varies a lot in voltage, sooner or later some leds will be failed!
On the other hand, for flashing, there are on the market real leds which can be supplied direct from a 12V power source without any problem. I'use them in a race car, too, just for a warning light. And guess what? They were flashing devices by them self! I didn't gave them the chance for being really flashers bulbs, but since our opinions are sprayed here, why not? You can discriminate those flashing leds than the usuals from a black small material inside of them.