Isolating constant from flashing signal

Not necessarily. In the USA it is still common to find the same light being used for indicating brake and turn

 

It is good that you can't do it. There are huge safety risks involved and you almost certainly will lose the license for using your car on public roads.
The "easiest" way (what is easy, what is hard is a matter of your capabilities) would be to feed the signal to a microcontroller, have the software in the microcontroller check for the duration of the signal. If it is steady for a period longer than the pulse period of the blink signal, do nothing.
If the signal shows changes within a time span a tad longer than the blink signal's period, activate an output to allow blinking (use an additional relay to pass the blink signal to the bulb).

But why for goods sake do you want to de-activate the brake signal? That is a vital indicator!

 

I disagree in this point .

 

As the others aske ... why? if for a vehicle on the road, totally illegal to do ... public roads, race tracks etc


of some sort of offroad use where no one would be following you ..... no reprogramming necessary ...

But before I give you the easy answer ... AGAIN, WHY do you want to do this ??