OK so, you can see where the BT module is located if you look at page 15 of the AV section of the service manual:
http://www.nicoclub.com/service-manual? ... %20Leaf/av
Referring to p15, there is an item B called out with a detail section showing item 12 which is the BT module with the connector you need to work with. Basically, you need to get to the top of passenger side rear wheel well to find the module. I'd recommend you print out that image, it's helpful when you are in the car trying to find it because it is buried deeper than you would think at first glance.
To get to the BT module you have to remove some trim panels. Referring to the interior section of the manual, page 43, bottom, where it says "LUGGAGE SIDE LOWER FINISHER":
http://www.nicoclub.com/service-manual? ... 20Leaf/int
On p44 there are several links you can click that describe how to remove each trim piece, once you get to the module, you can work on the wires in the connector.
1. I worked on the connector with the back seat still installed in the car, it can be done. If I recall correctly, it helped to fold the seat back down.
2. Personally, I used some solid core wire to jumper between the pins that you need to connect together, however, sometimes I have issues with buttons not registering intermittently, so make sure you do a good job with these connections...soldering the wires would be best, or maybe crimp connectors to ensure a reliable connection.
Also, I just used the mic that came with my head unit. Wasn't sure of the impedance being compatible or how to connect it to the car one. If you look through the AV section of the manual, you could probably trace out the circuit though and try it. I bet it goes through the BT module as well, but I don't have time to look it up at the moment.
Hopefully that's enough to get you to success...please let me know if you have any more questions and update folks here with any more notes that might be helpful. Cheers!