Arguably the easiest (and undoubtedly illegal, as are all "wig-wags") way to make a wig-wag flasher for your vehicle is with a standard SPDT relay and a thermal flasher.
However, with a few extra components, you can have a less power consuming solid-state alternative... along with the ability to adjust the flash rate with the addition of a potentiometer.
Of course, if you're after simplicity, then you can replace the 555 circuit with a thermal flasher and be done with it the whole thing.
If myself like to make things even more complicated than necessary, and whipped up a completely solid-state circuit...
It's a little "kludged" together, but the basic layout is as follows, with only a few changes... and I'm sure it could be much simplified.