The tail light works? Just the brake light is not working?
If the tail light works, that indicates the ground and running light circuit is good. You now make sure the wires under the seat are connected. Then start testing at the switches to see if the power is getting to them.
Here is a link to the wiring diagram in the Tech Section:
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1294779818