I won’t go into a lot of step-by-step detail here as there are plenty of resources (YouTube, etc.) to show you how to remove the gas tank.  Additionally, the T120 service manual instructs you to initially disconnect the battery (ground first).  Probably a good suggestion when working with a part of the bike that contains fuel and electrical connections.  My gas tank was full, so I did siphon all the gas (or as much as possible) from the tank.  Also, make sure the key is removed from the ignition during all these steps.  Basically, you remove the seat; then undo and remove the bolt at the back of the tank (See Step 1E for bolt location).  Lift the back of tank up approx. 3-6” to allow access under the tank. Pull breather hose away from (2) clips located on the underside of the fuel tank on the bottom edge (LH side), then pull the breather hose completely off of the bottom of the tank (LH side near front) - (Pic Highlight A).  Undo the electrical connections for the fuel pump and fuel level sensor under the tank (RH side) – (Pic Highlight B and C).  Undo and pull up the fuel hose (RH side) from the fuel rail – To do this, pull back on the locking tab, push buttons on each side (inner and outer) and at the same time wiggle the connector up (Pic Highlight D).  Some fuel will leak out of the hose, although the tank will not empty onto the floor as the opening at the bottom of the tank is “self-sealing”.  Lifting the tank up at the back (near area where the fixing bolt was removed), hold tank (completely so that you can safely carry it once removed) and slowly and carefully pull backwards to remove tank.  Set the tank on a piece of cardboard on the floor or workbench.

Again, I won’t go into a lot of step-by-step detail here as there are plenty of resources (YouTube, etc.) to show you how to remove the headlight.  Basically, you remove the two screws on each side of the headlight bucket and rotate the headlight up (to release hook at the top) and swing forward to free the headlight from the bucket.  Remove the large headlight plug and then CAREFULLY, remove the small connector at the back of the headlight.  I was not so careful and ended up breaking the latch on this plug which I ended up replacing (not fun!).  Remove the headlight and side aside.

Remove the two screws that hold the plastic cover (inside the headlight bucket), and gently pull the cover forward to expose the plugs/wiring.  The (2) turn signal connectors we will be working with are located on the LH side of the bike (inside the headlight bucket) as shown.

Locate the (2) turn signal connectors inside the headlight bucket (coming from the turn signal switch) as shown in the first picture (and highlighted in the previous step).  Disconnect these (2) connectors.  Note how the turn signal wire harness is routed from the LH grip so that you can reassemble using the same path (Take before pictures with your smartphone/camera before disassembling) .  Carefully pull the turn signal wire harness through the opening in the back of the headlight bucket and allow it to hang from the LH grip on the LH side of the bike (as shown).

Cut all the connectors of the wires on the STS unit.  These ends will be connected to the end of the 36” extension cable using solder or (in my case) solderless connectors to save space under the tank.

Completely remove the black outer sheath from the entire length of the 36” extension cable (obtained from STS or reseller like A&J Cycle): Starting at the end of the black outer sheath where the wires exit - Use a sharp wire cutter and carefully run the jaw of the cutter along the inside of the black outer sheath, so that the sharp cutting jaw is facing out and the smooth rounded side of the cutter slides harmlessly along the wires inside.  Go slowly and be very careful – If any wires are nicked you will need to replace this extension cable.  The black outer sheath on mine was quite thick.  Push away from yourself and take your time.  You can also pull on the jacket to tear it open once you get a cut started.  This will step is required to reduce the diameter of this cable and also allow you to eliminate the RED and BLACK wires from inside this extension cable in a later step.  (Note: These RED and BLACK wires are no longer needed as the power/ground connections will be made under the seat – However, save these wires for use later to make wire leads.)  Eliminating the black outer sheath will also make this extension more flexible which will make it easier to route under the tank and into the headlight assembly.