The decoder of the 3769 is likely a 6090. This decoder has no + pole.
So you can create a + pole with 2 diodes (1N4148 are strong enough), as follows:
-Connect the + (anode) of the 1st diode to the loco chassis (ground)
-Connect the + (anode) of the 2nd diode to the pick up shoe
-Connect together the cathodes of both diodes (the cathode is marked with a circle on the diode body)
-deconnect the wire of the light that is connected to the chassis, and connect instead a wire between the light and the 2 diode cathodes.