Those two black wires going to the front of the locomotive are for the headlight but if they are like the non-DCC ready older Big Boy, they are also connected to the front set of drivers for electrical pickup. The way you have wired them to the decoder means you have probably toasted it. You'll have to further disassemble the engine to get to the front set of drivers and rewire the headlight.
You need to have a VOM and trace the wiring on an engine of this type because yours may not be the same as the two I have put decoders into.
If you are anywhere near a Harbor Freight store, they should have VOMs for $2.99 or so (prices fluctuate weekly).
HO RP 25 Wheels
Rivarossi 4-8-8-4 Big Boy DCC Ready
Printed circuit board not in engine. I am trying to install decoder in unit hard wired. I have orange to Right side, motor gray Left side. Rear Eight had black to tab on gear box. Front eight red to tab on gear box. Two black wires ran to pilot at front for Light - I think. Two black to tender backup light. I put decoder in between all using JST plug and had nothing.
Yesterday after doing this, I installed a jumper into JST plug and using DC transformer got power using front pilot and rear pilot trucks. Today - nothing. NO DC - No DCC. Help!
Thanks
Stardog1