This isn't really a "Ask Volition!" question...maybe better off in the help section, it's just an in-game thing that other players can answer.
Looking around (google), nobody's found a customizable Rattler spawning anywhere; a quick look through some .xtbl's would confirm if one exists but isn't spawned, though I've never dealt with vehicles, or even spawning .xtbl editing.
If the car exists already, is customizable, et al, but isn't set to spawn anywhere, it'd just be a simple .xtbl editing.
[...]
TEMPORARY SOLUTION FOUND!
+80 XP...
Just checked
IdolNinja's Additional Cheats mod, and it includes a "
giverattler" cheat, and in his mod it won't flag your savegame as having used cheats (I used it when the SRIV update broke the weapons system to give me back some unique weapons that aren't found anywhere, but that's since been fixed in the most recent patch).
Try that, see if you can customize it (first, with
out his Customization mods), and if so, someone just needs to make it spawn somewhere. That'd be for the mod requests section.
If it works, be sure to click "Thanks" on Idol's mod's release post.
Then, feel free to request it in the mod request section for SRIV talks if it hasn't been already.