As you probably know, your trunk (roadster) or hatch (coupe) is electronically released. I've known for a while that there was a manual release somewhere but I couldn't figure out where. So when the answer came up on Vortex recently, I figured I'd add it to here for everyone's benefit:
-In the roadster, the manual release is located in the small compartment behind the passenger seat.
-In the coupe, the manual release is located under the cupholders. You must remove the rubber pads on the cupholders and unscrew them to access it.
Hope this helps you if you ever need to get into your trunk for the jumper cables!