The brief
The Jetta had crossed 120,000 km and the owner was hearing a high-pitched squeal under aircon and alternator load. He brought it in before the belt actually let go.
The accessory belt is the one that drives the alternator, the aircon compressor and the power steering off the front of the engine, and it's kept tight by a spring-loaded tensioner running on a pulley. As the belt ages it glazes and cracks, and as the tensioner's bearing wears it develops a wobble and stops holding the belt steady, which is the squeal under load. A belt that's started squealing on a high-mileage engine, with a tensioner that's wobbling, is a set on its way out, and a snapped accessory belt leaves you with no charging and no steering assist.
The diagnosis
On the lift the tensioner pulley showed a visible wobble on its bearing, and the belt itself had a glazed face with surface cracking, exactly what an old belt looks like.
Doing just the belt would leave a worn tensioner ready to be the next problem behind the new belt, so it was a set job, the belt and the tensioner together, with the idler pulley while it was open.
The work
The tensioner was released, the old belt removed, and the tensioner swapped for a new VAG-spec unit along with the idler pulley. A fresh belt went on, routed correctly, re-tensioned to spec, and the running checked with the engine warm.
A road test confirmed the squeal was gone and the belt drive ran silent.
The outcome
No squeal, no wobble, and a silent belt drive across the rev range.
The Jetta went home with the front of the engine quiet again. The accessory belt and its tensioner 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.