When the solder has cooled, use your track cleaning eraser to remove the oxidation which resulted from heating from both sides of the spring. Use the smallest amount of solder possible to make a good connection to the top of the cap. There is no room for solder on the edge of the cap . Test fit the brush cap without the spring and brush to be sure there is no contact with the metal motor housing. Remove with a small file, any solder that may be causing a problem here.
Test fit the other brush cap in a similar manner.
Solder the RED decoder wire and the RED wire coming from the rear of the locomotive trailing truck to one of the tender pickup contact springs. The front light board has an incandescent lamp glued to it with the lamp leads positioned so as to contact the frame halves. If you want the light to always be on, just leave it as is and re-install it between the lower frame and the boiler weight as it originally was. If you want to control the light with the decoder, solder the white wire to the lamp lead that would have touched the left frame half for power. The White wire should be routed and tacked with GOO as shown in the picture .