The brief
The Jetta had crossed 120,000 km and the owner was hearing a squeal on cold starts and under aircon and alternator load, with the belt clearly slipping when the demand came on. He brought it in before the belt actually let go. The accessory belt drives the alternator, the aircon compressor and the power steering off the front of the engine, kept tight by a spring-loaded tensioner running on a pulley, with an idler to route it. As the belt ages it glazes and cracks, and as the tensioner and idler bearings wear they stop holding the belt steady, which is the squeal and the slip. A belt that's started doing that on a high-mileage engine is a set on its way out, and a snapped accessory belt leaves you with no charging, no steering assist and no water pump drive.
The diagnosis
The belt was glazed and cracked along its rib face, classic age-and-heat fatigue. The tensioner had a wobble on its bearing and the idler pulley spun rough. Doing just the belt would leave two worn parts ready to be the next problem behind it, so it was a set job, belt, tensioner and idler pulley together, rather than back here in a few months for the rest.
The work
The tensioner was released, the old belt slipped off, and the tensioner and idler pulley swapped for new genuine VW-spec parts. A fresh belt went on, routed correctly through every pulley, and re-tensioned to spec. A road test confirmed the squeal was gone and the belt drive ran silent across the rev range.
The outcome
No squeal on a cold start, no slip under load, and a silent belt drive across the rev range. The Jetta went home with the front of the engine quiet again and ready for another long stretch. The accessory belt and its pulleys wear together, and a snapped belt strands you with no charge and no steering assist, so doing the set on the first squeal kept it to a tidy job.