This mod modifies the GTAIII timecycle to be a darker and Rainy one. There will be several differences that you can notice. The timecycle has frequent rains during the day and the nights are even darker. The sky remains more dark till morning.
Installtion
Put the timecycle.dat in GTAIII Installation directory\data\ folder.
Be sure to make backups. Let me know if you face any bugs.
Enjoy.