If the rear turn signals are mounted in bags, then they are aftermarket. Generally aftermarket signals that use dual filament bulbs, like an 1157, will have two small contacts in the bulb holder socket. These are positive contacts and the application is meant for running lights and turn signals. These type bulb holders rely on a frame ground and generally, one has to be provided. Take a piece of wire and unscrew the bulb holder and wrap the end of the wire around the screw and retighten. Then connect the wire to a frame ground. Now, you will have a circuit for both filaments on the 1157. To connect them, don't pay much attention to the aftermarket wire colors. They will generally use black, white and/or red for these wires although both will be hot (positive). First, note that there are two filaments on the bulb of dissimilar length. The longer filament is the brighter and should be used for the turn signal. Just note how the bulb orients when you put its bayonettes in the holder and twist it. The wire associated with the longer filament will be used for your turn signal connection.
At the rear of your wiring harness is a connector. It will have a BLUE wire, a RED wire and a BLACK/YELLOW wire and either TWO GREEN or TWO GRAY or A GRAY and a GREEN wire. You can splice into the BLACK/YELLOW wire with the GROUND wires from your turn signals OR connect them to the frame; probably where the turn signals mount. It doesn't much matter. Just make sure the ground connection is secure and don't try grounding on paint.
The GREEN and GRAY colors are used on most all Kawasakis for turn signals. Sometimes one of each. Sometimes two of the same color. Run one of these wires to each of your turn signals. At this point, you have a 50/50 chance of connecting them correctly as far as right to right and left to left. Make temporary connections because you may have to reverse them.
To connect the OTHER filament (lower wattage and shorter) on the 1157, connect the wire associated with it to the RED wire from your harness. This is the running light wire on your brake. On older models, this power is switched on when you switch your headlight on and newer, it comes directly from the ignition switch. In any case, it will provide a running light. For reference, the blue wire in the harness at the rear is the brake light wire.