Thanks for the polarity tip! I got lucky (50% chance on each one that I'd insert incorrectly...both work!). The bulbs from the seller on eBay fit into the holder (plug) really tight, so at this point, I'm not too concerned about them falling out, but will keep an eye on them and maybe check after I've put on some miles (and vibration). Cheers!
Short video (1 of 2) showing "flash"
Video 2 of 2