Power off the bicycle and remove the battery.
Using T10 Torx driver, remove two bolts that hold motor cover on
Loosen rearmost motor mounting bolts with 5mm allen wrench - the bolts enter from either side and thread into the motor casing. DO NOT LOSE THE LOCKING WASHERS ON THOSE BOLTS- sometimes they stick to the paint
Before you remove the motor, you will want to remember how the wires are routed around the motor mounting points. It is best to take a picture so you can route it the same way upon install. There are 5 wires that are routed from the frame and make a connection to the motor unit (list/diagram at the bottom).
Now you can access all motor mounting bolts - remove the remaining 4 with a 5mm allen wrench. Leave the rearmost bolts in place loosely to support the motor. DO NOT LOSE THE LOCKING WASHERS ON THE MOTOR BOLTS- sometimes they stick to the paint.
After removing the forward motor bolts, tilt the motor to gain access to the wiring.
You can now see the large blue Harmony plug. This is the likely culprit with the failed solder joint, and the cable which you'll be replacing. After you route the cable through and connect it, I would suggest powering up the bike to confirm that you have power to the shifter, just to make sure--then you can reinstall the motor.
Installation is reverse of removal.
Make sure to route wires/harnesses the same as they were before. The wires often have a memory to which way they were curved, so following those will make installation as easy as possible. Be careful not to pinch or crush any wires/plugs when the motor is installed. There is no force needed to push it in place if the wires are routed correctly. If force is required, a crushed wire or plug will result. Be patient. Inverting the bicycle might make this easier as gravity will help keep the wires in the proper spots as the motor is laid in place.
The easiest way to route the Harmony wires is as shown below. The yellow wire is from the shifter and passes over the mounting bolts and up into the battery dock void. The red line is from the rear axle, through the grommet, over the mounting bolts and into the battery dock void. There is enough space in there to place the blue and black connectors.
NOTE: The blue and black connectors for harmony are under the battery dock. There is still a juliet-style push/pull connector and bullet connectors for the Harmony power under the motor cover.
One the motor is secure, you will reinstall the cover, chainring sprocket and crank arms. Check chainring bolt tightness before installing spider/chainring as access is currently very easy on both sides. When installing the lockring (turning to the left to tighten), make sure it is very tight to prevent creaking.