There may be a better way to "flip" a mirror in GTR2, GTL, RF, etc. but this has worked for me countless times (assumes that you have a working knowledge of 3DSimED3 operations and do back ups before modding) :
To be more precise:
Mostly the mirror is named "mirror.gmt" with the texture "mirror.bmp"
If you don't find it in the Objects.gtr file ... it will be also often to be found loose in the Cpit.gtr or/and in the XXXXX_cpit.gmt
If you have an older 3DSimed ... in the Edit tab, choose "Edit Materials" ... click on the mirror in the opened window, click Edit then Rescale Tex and just change the Scale U from "1000" to "-1000" and click the necessary OKs .... then Export ... choose GTR2 ... Save Model data, click OK without changing anything and replace the original using the same name and confirm.
It happens mostly with Rfactor conversions to GTR2 ... and only the mirrors are horizontally inverted.
NB: If you're not used to it, you'll need also nearly always the tool GEditor as the mirror.gmt and mirror.bmp will often only be present in a .gtr file.
No problem, @Tomal ... you may send me the file in a private conversation.
GEditor works fine but there's one thing you need to know about it.
If the 1st file of this .gtr file is a dds or tga or any image file ... this won't cause any problem for exporting any following file.
But if this first file in the .gtr is a .gmt file ... you won't be able to export anything.
So open first a XXXX_tex.gtr file .... then from the software itself open the Objects.gtr file.
You'll need when choosing a gmt to click OK when it is asked .... and then you'll be able to export it.
You may alternatively extract (or compress) GTR and many other files with 3DSimED.
Please accept my apology for my previous "Shorthand" answer to your question; I hope that this is a more "precise" explanation. I am not a paid spokesman for 3DSimED3 but I AM a staunch advocate of purchasing software that makes daunting projects considerably easier for non-professionals like myself who simply wish to create or customize simulated objects. Inexpensive programs such as 3DSimED, ZModeler and BobsTrackBuilder have made it possible for me to create and/or modify objects from Grand Prix Legends, NASCAR Racing, MS Flight Simulator, rFactor (1&2), Grid, F1 20xx, Forza, ALL of the Simbin titles, etc.. Once the idiosyncrasies of these apps are mastered, they ARE the easier way to modify many objects. I am as pleased to contribute to brilliant programmers such as Dave Noonan, Oleg and Brandon as I am to the major game providers.
The following screen captures may best illustrate my methodology:
STEP 1 - Click "Expand>GTR:
STEP 2 - Create new folder:
STEP 3 - Click "Import> Select Import Filename":
STEP 4 - Right-Click on object, then click "Material"> "mirror" from drop-down list:
Yes, many thanks to both of you ! I must admit I was and is totally lost on this one. Your precise tutorial will help definitly if I ever encounter the same thing again. I just have to find a way to make GEditor work.