This shows completed installation with the M1 decoder. Notice the decoder wires are cut to a length that allows the M1 decoder to just hang off the end of the circuit board.  When the cab is put back there is plenty of room inside for the M1 decoder.  Do not leave the decoder wires to long or you will end up trying to coil the wire up to fit in the cab. It is easier to not have to do that by having the length as shown in the photo.