Note: this can also be used for Sony NEX-3, NEX-C3, NEX-5 and Alpha series
What I needed:
Opteka RC-3 Wireless Remote Control for Sony
NEEWER® Timer Remote Control RS-60E3
2.5mm Female to 3.5mm Male Headset Adaptor
First I had to open the remote. While it looks like the plastic splits apart, it does not and the face peels of like a sticker. It's easy to push it up from slit that the battery came out of.
Slowly peel off.
Remove screws from circuit board and pull it all apart.
I cut off the male connector of the headset adapter and exposed the wires.
Drilled a hole in the back cover of the remote.
Using a multimeter I found the two connections on the intervalometer that would trigger the switch. I then found that the two colored wires on the headset adapter corresponded to the right connections on the intervalometer. I traced the connections from the shutter switch on the remote to the back of the board then threaded the wire through the hole and soldered it on to the circuit board.
Carefully reassemble and add a dab of glue to prevent the wire being ripped out.