I have a single motor 44tonner and a cap pack with two wires. The cap pack instructions simply say to connect to the blue and the black wires of the decoder pcb pads, but Soundtrax showed me different pads on my S4. I have read that the 44 Tonner's decoder/PCB isn't by Soundtrax which figures considering the 44tonner doesn't have sound !
I'm also fitting a Digitrax DZ126 as the oem seems to have died.

The blue wire on the PCB is the V+ output for functions such as lights, etc.  It is one side of the keep alive connection. Power the loco and use a DC setting to find the V- side of the rectifier bridge on the PCB for the other keep alive connection. That would be one of the pads of the D1 - D4 rectifier array.

Or you could just connect it across the +/- LED connections on one end of the PCB. The blue keep alive wire goes to the + connection.

Len