Your owners manual for the car should tell you what that plugin is rated for -
or you may have to explore your fuse box(es) a little to what what fuse they put on it. If its a rating you can use, say 10a, I'd try it out before making permanent mods just to be sure it'll work. The wires can get hot if they're overloaded, so be careful, but I'd expect a fuse would keep you from doing any real damage.

Part of this depends on what you are planning on charging too, if its a pr of 6S 5000 lipos, you're prob out if luck.