Well I don't think you should hit the rebuild button, I don't know exactly what it does but maybe that button makes you're previous action undone. Try doing it without the rebuild button and only the save button. Also I suggested the car trainer because modded cars are less likely to appear in traffic so it could be the case that they are in the game but you just didn't see them in traffic. Did you see the cars you modded in default form (indicating that indeed the car mods don't work?)
If you really can't figure it out .. just send me a zip file with all the car mods you want (upload it on mediafire or something and give me the link via PM). I'll put them in for you and return the vehicles file to u.. all u have to do with that is replace the orginal.
If you'd like it this way, make sure you delete the file immediatly after recieving the updated file from me, wouldn't want to piss off any carmodders for hosting their files.
*Edit* nevermind the pm, email me at email@example.com
I don't think you have PM rights just yet