Bruno,
Remove the wire from terminal 5 on the voltage regulator and ground that end of the wire. Turn on the master switch. The lamp should illuminate. If it does, the above test proves that the circuit breaker and lamp are good.
If the lamp does not illuminate, further testing is required. I like to use a 12 volt test light similar to this one:
http://www.autozone.com/autozone/acc...er=186543_0_0_
Connect the test light clamp to ground and test the output of the circuit breaker with the test light probe. Of course the master switch needs to be on for this test. If there is voltage at the circuit breaker, then probe the lamp. If the lamp is getting power but does not illuminate, the bulb or socket could be bad. Do not replace the circuit breaker unless testing proves that it is bad.
Joe Gores