One of the major provisions of the Magnuson-Moss Warranty Act is that warrantors cannot require that only branded parts be used with the product in order to retain the warranty:
“No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer's using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade or corporate name” (15 U.S.C.A. 2302).
However, in the case of "the warranty covers the main EV battery, and messing with/adding wiring explicitly voids the warranty", they could argue that you did something really BAD to the system (and would have to prove it)-- but since the lighting on many hybrids is still run from the 12V electrical system (using a standard starting battery and 12V alternator). I'm not sure if the Kia EV also has a standard starting battery, but obviously the lighting system would still be 12V (and you'll still probably have a 12V "accessory socket", which is simply a renamed cigar lighter socket without the cigar lighter).
You could also always install a rear fog lamp and pull all the wires where you want them but don't hook them up permanently-- and use a cigar lighter plug to plug it in and unplug it as necessary. (Very bad UI, of course, and you'd need a telltale to be sure the lamp was really powered on and not just plugged in (like from having a blown fuse or just a loose connection).)