Here is my take on this. Referring to the attachment, Fig. 1 is what I think you need. Fig. 2 is the truth table for what you need. In fig. 2, D is a signal that is high when the flasher is "engaged" (either left turn or right turn). F and B are you signals as described previously, and L is the lamp signal (1 means ON). Don't worry about how D is derived for now. From the truth table, you cna see that when D is 0 (no flasher engaged), the lamp signal follows the brake signal regardless of the F signal). When D is 1, the lamp signal follows the F signal regardless of the state of B.
Now, how do we derive D. The best way is to use a retriggerable positive- edge triggered monostable. Set the monostable period to be a bit more then the high period of the T signal. If the flasher is flashing, then the monostable will get retriggered before it times out, and the output will be a constant high for as long as the flasher is flashing. As soon as the flasher stops flashing, the output turns low.
If you are willing to experiment a bit with values, you can get by using the ciorcuit in Fig. 3. In this circuit, everytime the flasher is high it chrages the capacitor. When it is low, the capacitor discharges through R2. If there is enough voltage across the cap, the transistor collector will be low. It will turn high when the flasher turns off. You will notice that the D signal is inverted wrt to teh truth table. This can easily be remedied.
So, now you have your D, T, and B signals. You can now proceed with the logic design, and power driver. Before going further, do you think that this is what you were asking for? Secondly, is it enough for you to work on?
Don't know exactly what your electrical level is. Don't want to take the fun out of it completely for you. However, I am willing to help.
Jem