I cannot believe you are unable to do this, its stupid. Some cars i use with tcs and others without and every time i want to switch it on or off i need to go into a test drive to do so. Unless im stupid i cannot seem to find in the menus where to do it?!
Highlight you GT in the hopper press A then scroll down to assists
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Knew there must have been something, my stupidness then 
Cheers mate
I just found this a few nights ago myself. I found it purely by accident.
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Same here, accidentally clicked on my own name and there it was.