You're right, cars don't see bikes. On all of my bikes, I install two white LEDs up front to give a "triangle of light" that distinguishes me from a car, and install a flashing red LED strip on the back that lights up with the brake light.
I'd need a second person to hold my brakes while I take the picture, so they're kind of hard to see, but on the Bonneville, the LED strip is on the bottom of my license plate. On the CB, it's right below my Givi rear rack. The strip is flexible and has a 3M tape backing, so it conforms to the shape of whatever you stick it to. The flasher gives a quick strobe, then a slow strobe, then stays solid.
Here's the links to the LED strip and the flasher module on Ebay:
https://www.ebay.com/itm/Custom-Dyn...=312433727106dd755b3a6bf54c0db93b1d22c7345977
https://www.ebay.com/itm/GS-100A-Ca...sher-Module-/273139088322?hash=item3f985bbbc2