When replacing tail lights with "tail-light-assemblies" containing LEDs, the amber turning-indicators will not flash because the LEDs do not take sufficient current to operate the "flasher" unit in the cabin.
 
Here is a typical replacement containing 2 red STOP LEDs and 3 AMBER TURNING LEDs:

The 3 LEDs on the PC board above
for the "Turning Indicator" are in series.
Normally the yellow and white leads are connected
to the "turning Indicator lead" (from the cabin) and chassis.
But of the flasher unit in the cabin does not "flash,"
you need the Multivibrator circuit below to make the
LEDs flash.

Connect the yellow and white leads to the flasher circuit

Connect the yellow and white of the tail-light assembly to the "lamp" holes on the PC board and connect the +1v to
the "flasher" lead and GND to chassis

The three Turning Indicator LEDs can be placed in series as in the tail-light photos shown above or they can be placed in Parallel as shown in the following circuit:

When placed in series, the total current will be less than 30mA and when placed in Parallel, the total current will be about 90mA.
Both arrangements will produce the same flash-rate but the parallel connection will allow the LEDs to turn off fully between flashes.