12v can be found on both pins of the through-hole pad marked Fan, however I'm not sure whether this is regulated or unregulated 12v nor whether the controller is fussy about it. If this is suitable, I'd suggest soldering a 2-pin header through the holes, then soldering the 12v line to a jumper and plugging the jumper onto the header. I don't know of a good place to get GND (that is, so it is easy to undo). You could splice one of the cables or solder onto the back of one of the sockets (perhaps the serial D socket), but most seem to simply opt for soldering a wire to the pad ("collar") around one of the PCB screws.

I thought there was a 2-pin through-hole pad marked I2C or IIC towards one corner of the MK2A board somewhere. Maybe I'm wrong and there is only an IIS pad, not an I2C pad as well.

It might be useful to create a new album on riocar.org with hi-res photos of the front and back of each PCB board including the add-on boards - that would have certainly helped with the creation of the memory manual.

[Edit: fix spelling and clarify]