should have mentioned - it's a 2004 passat

Sounds like a classic case of water logged convenience control module to me. It's situated on the passenger's side floor under the carpet/sound deadening and gets wet when either the pollen filter housing or plenum drains are blocked and water can get inside. Another favourite are the connections in the passenger side A pillar which also get corroded by water ingress.

Check the floor on the passenger side. Sometimes the outer carpet feels dryish to the touch, but they almost always do. Remove the plastic sill and kick panels, lift the carpet and sound deadening and check for wetness. Also check the multi plugs on the holder on the A pillar whilst you're there. It's usually the grey coloured one that goes nice and green inside from corrosion.

You may be lucky and a good cleaning of any connectors that have corroded may cure it but you WILL need to find and cure the root cause or the water ingress and if the carpets and sound deadening is soaked, the whole interior removing and the carpets etc drying out otherwise you'll have problems with the interior misting up and a stale musty smell.