found this by ycming fan belt
Jack up the driver side, take the wheel arch off, then u will see the alternator being held on the 2 bolts,.
Losen them, the u can adjust the tensions of the belt by pulling the alternator back towards the rear of the car [FONT=inherit !important][FONT=inherit !important]
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It might help if you stop calling it a fan belt, as there's no such thing in the Cinq, the fan is electrically driven
If the lights are dimming it will be the alternator belt, try as ming suggests. Best to post a thread in the Cinq/Sei section though for more thorough answers