First you need to make sure you are getting power to the lamps.
You may also have two separate problems.
Common problem:
The turn signals and brake lights have a common power source, the third fuse. Is there anything else not working?
Turn signals:
Since the hazards are working, the grounds must be ok. Hazard lights get power from the brown wire circuit, whereas the turn signals get power from the green wire circuit.
Someone else recently had the same problem. Some trouble shooting steps can be found in the following thread.
http://www.mgexperience.net/phorum/r...271#msg-751271
Brake lights:
Have you checked for power to the sockets? Have you checked the input and output of the brake light switch? High resistant connections can cause voltage drops. The following thread shows that a failing brake light switch caused the problem.
http://www.mgexperience.net/phorum/r...595#msg-728595
Wiring diagrams are available at
http://www.advanceautowire.com click on stock schematics.
You will need a copy of the wiring diagram and a meter or test light to correctly diagnos the fault.
In the future, please include the model year when requesting assistance. Not all Bs are wired the same.