Installation:
To open IMGTOOL => File => open => gta3 (the list of files will open)
Further: Edit => find (write the name of a replaced file, in this case replaces "sultan", therefore we write: sultan.dff => replace (we choose a file from archive "sultan.dff" => replace, similarly we arrive with a file sultan.txd))
After both files are replaced we press: edit => rebuild archive 
We wait the termination for "archive reorganization", we press file => exit
We start game => we go for a drive :)

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Pleasant game!