OK, first question, does your Neutral light work?? This is an important question to prove that electricity is getting past the signal fuse. The next thing to look at would be either of the two brake light switches and measure with a voltmeter for 12 volts on the Green/Yellow line with the brakes applied. The G/Y line becomes the yellow line after the brake switches and goes to the CMU. If the switch is passing voltage then your next place to look at will be the CMU but before that find the connector for the tail/brake light assembly. This is a 3 wire connector with a black, blue, and yellow wire coming from the assembly. The other side of the connector has a black, blue and white, and yellow and black wire on it. With the brakes applied look for 12 volts on the yellow wire just to prove that voltage is indeed getting that far, there is a remote possibility that both bulbs have their brake light filaments burnt out...