Here is an easy installation project for anyone wanting to add a 12-volt electrical socket in the trunk. I first learned of the possibility on page 208 of the 2013 S60 Owner's Manual. Unfortunately, I was unable to find any information on Volvo's accessories website.
During my research, I removed the trim cover on the left and checked the fuse box in the cargo area. Sure enough, slot 6 is populated with a 15-amp fuse.
Now why would Volvo put in a fuse without running a cable of some sort? As I reached below the trim, I felt a cable with a connector of some sort taped to a wiring harness. I removed the masking tape holding the cable. Sure enough, I recognize the connector as the same type used by Volvo to connect a power outlet on other vehicles such as the XC60. Learning that Volvo already provided the cable, all I have to do is to drill the hole, screw in the power outlet and plug it in!
I purchased the Power Outlet (p/n: 31324708) from Tasca ($17.53). I chose this location since it offered enough clearance behind the trim to accommodate the depth of the socket and within the reach of the cable.
I removed the four plastic screws (10mm hex socket) on the trunk floor, one clip on the upper left, and slid the trim away from the metal frame. There is one more clip on the top of the trim that I could not remove without breaking it so I decided to leave it. I was able to move the trim enough to work while leaving the last clip still mounted. I lined the trunk with a garbage bag to catch the plastic fluff and proceeded to drill.
The installation instructions recommended using a 27mm drill bit, but I used a 25mm. Once the drill bit was through, I used a box cutter to make a small notch, about 1/4” wide and 1/8” below, to match that on the socket. I twisted the socket through the freshly drilled hole. The fit was snug, which was good. Once it reached the end, I pressed the socket so it was right up against the wall of the trim. Using an oversize wrench, I tightened the plastic nut onto the back of the socket. I moved the trim back in place while leaving enough space for my hands to connect the cable to the back of the socket. I tested the socket for power before putting the trim piece back.
All-in-all, an easy and inexpensive installation. It took me about an hour from start to finish. I spent half the time deciding on where to place the socket. YMMV!