The only solution needs 3DSimed.
You have to open the GMT in which the mirror GMT is included ( in the cpit.GMT generally, but might be in another GMT as a cockpit_body GMT for example, but I don't know for this DBR9 )
In Edit Tab select it and click "Rescale Tex"
Change only the Scale U 1.000 for -1.000 click OK and save
That's done, simply replace the modified file in the cpit.gtr file and it should mostly work.
You can do this for any GTR2 car.