The motors are almost certainly isolated from the chassis, but as Engineer-Joe says, check with a meter.
I consider the center pickups 'positive', and the return via chassis/wheels 'negative'. The pair of positive leads should be tied together and soldered to either the Left Track or Right Track pad on the board. It doesn't matter which you use. The negative leads should be tied together and soldered to the other Track pad.
Similarly, both yellow motor leads should be tied to the 'Motor +' pad, and the blue leads tied to the 'Motor -' pad. Again, it doesn't really matter which leads go to which pad - you can define 'forward' in the decoder configuration to match the physical hookup.
I have that decoder installed in a number of my engines, including one I just recently installed in a Williams GP9. I think you will like it.
