If a car does not appear in the menu it is usually an issue with the .car file, check that the "HDVehicle=" and "Graphics=" lines point to the proper files and that those files are present; also the manufacturer (which is the title of the car in the menu) and class lines.
If the car is listed in the menu but does not appear, check the .cas file for the "MASFile=" lines and make sure all files listed are present and in the path. If all appears well, select the car even though it doesn't appear in the spinner and attempt going to a track, the game will probably crash; now check the trace file, near the bottom should be an indication of the problem (usually missing wheels or driver parts, which are often loose files not in the .gtr files, so easily misplaced). And double check all spelling, even a space between an "=" and a word can cause problems.
If this mod was converted from GTL the .cas may call
"MASFile=STEX.GTL
MASFile=TALENT.GTL"
if these files were not included with the mod you will need them from GTL.