The wires running under the seat to the rear tail light are red and blue with a ground of black with yellow stripes. The thin bullet connectors should be cleaned and compressed to ensure good contact. Inspect to be sure nothing is chaffing and shorting to the frame.
Red wire and ground get you the tail lights while blue is the brake lights.
There are 2 bulbs in the housing that should be dual filament bulbs. If they are LED's, take them out and put regular bulbs in there for testing purposes.
If you have the wrong bulbs (single filament) then it may not light at all. Check your connections under the seat and also check that your bulbs are seated properly and getting voltage. Work your way backwards from the tail light while checking voltage is present. It could be a bad ground connection or a bad power lead.