Nevermind, found the solution, i had to turn the mods on with the Workshop installer

Where can i find it ? I looked inside the Steamapps directory and the game's directory with no results

Check if the workshop_uninstall directory contains the file 3026839808

And yes, i was in the garage when i searched the mufflers

I had the Remo

What cars did you try it with? Btw, car must be in garage when accessing catalog.

Yes

Either way it should work after reinstalling the mods.
Are you looking at the right place: Catalog-> Body-> Misc ?

Still not working, cars probably got overwritten by an update

Yes, i did installed CGT, i have no other mods, i'll try uninstalling the mods and verifying game files

Did you install the required mod? Unless car files are overwritten by game update or other mod (see previous comment), it should enable mufflers in catalog for all vanilla cars.
When in doubt uninstall all mods, verify game files, reinstall mods.

I try to update my mods as fast as possible when affected by game updates.
Btw, with Custom Workshop Installer it's easy to see if a newer version of an installed mod* is avilable. (* requires mod author to include a specific .txt file)

I don't have any custom mufflers, is it reserved to specific vehicles ?

Its helped , thanks.

I guess you installed "Axis RWD - Standalone Item" on top, as it replaces the Axis_chassis.java and by that disables the changes. Get this instead "Axis RWD"

When i picking axis , i don't see mufflers in the catalog. But when i picking any another car , everything works fine.