Cleaning under the hood of the '05 roadster I discovered this wire and connector at the battery.
Red wire connected to the + and black wire to the - post. Not jury-rigged, there is a nut on both posts
specifically for this whatever-it-is. No inline fuse, just the male/female connector on the end.
Its too short to reach just about anything so apparently its just a tap on the battery, but for what?