Hi
The silver tab on the left hand brush strip is located behind a tab on the metal chassis, this is connecting left strip to metal chassis and wheels. I've marked this with a red arrow on the image. This connection
MUST BE REMOVED. Ideally slip a length of heat shrink tubing over the silver brush strip tab to act as an insulator from the metal tab or cut the silver tab off completely. Once removed test with a multimeter set onto continuity or use a buzzer and battery or even a lamp and battery with fly leads to test that the left hand strip is isolated from all motor chassis metal and wheels. If it is proved then continue. If you get a reading, buzz or the lamp lights DO NOT PROCEED and investigate the connection cause and remove it, then retest.
Remove the spade connector on the right hand brush tab and cut it off, this is the wire I've shown arrowed Blue and make an in-line soldered joint with the end of that wire to the decoders red wire. Cover the joint in Heat shrink tubing or use PVC insulation tape over the joint. Cut the white wire or very carefully strip a little of the insulation away exposing the inner copper wire, where I've show the Green arrow. If cut then join the two cut ends back together and to that joint or the area of bared copper connect the decoders Black wire onto it or the joint. Solder the joint and cover the joint with PVC tape or heat shrink tubing. Solder the decoder Orange and Grey wires to the now insulated from wheels and metal chassis brush strips.
Also remove the little brown capacitor that's connected across the two brush strips. Its not needed on DCC.
Position decoder and tidy wiring ensuring it cannot get trapped or foul any moving parts. Test loco on Programming track and set address to whatever is needed. If OK test loco on main track. If it runs the opposite way to the console setting reverse the Orange and Grey wires or use CV29 to alter normal direction of travel.
BTW I would also give the motor copper Comm. a good clean as it looks very black and covered in Carbon from the brushes. Use a soft fibre brush or a cotton bud dampened in Isopropanol Alcohol (IPA) or Meths. DO NOT USE anything abrasive on the Comm. Also worth check is the condition of the two carbon brushes and replace them if badly worn - Spares are available, Peters Spares is one such supplier and even eBay has sellers too.
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