MG Car Repair: MGB, switch failure, test light

Question
I have a 66 MGB and the tail lights and brake lights sporadically work and don't work.  I've checked fuses, have the wiring diagram and have checked all connections -- any other ideas?  Thanks

Answer
One of the hardest things to fix on any car is something that quits working at times.
Other than getting lucky you need to do your testing when it is in the failed mode. Only work on one circuit. If the tail light is out quickly look at the front parking light to see if it is out also. This would indicate a failure of the power to the red wire off of the light switch. Then you should either test the red wire at the light switch or if it is only intermittent, put a test light on the red wire at the light switch so you can monitor the power coming from the switch while driving. If you see a lose of power at that red wire then it is a light switch failure.
If you use that method of testing you can't fail to find the problem on any circuit.
Howard