First step of the retrofitting
I've placed everything in the small rear pocket on the backpack.
Leds are connected to an Arduino Nano sewed inside of the pocket. Also the power is a small smartphone battery pack placed inside of the pocket.
The switch is made with the zipper, by closing the zipper the Arduino become connected and turns on. Opening the backpack zipper turn off the device.
Everything is connected using conductive threads, which was quite difficult for me... but it was also funny.
Here is a small video with lights and zipper switch:
The leds are powered using POV techniques to reduce the power consumption through Arduino pins.
Next step, turning led signs
I will add an accelerometer/gyroscope (I've got some GY-521 here) to detect the inclination of my backspine (left or right) to determine the left or right led sign before turning when I'm on bike.