Multifunction Switch problem after major service

We just had the '99 Passat in for major service related to the Oil Sludge problem on the 1.8Turbo. Needless to say, you MUST present all of your receipts for oil changes through the life of your car or they will not validate the extended warranty on the OS problem. Scope of damage for our car was Oil Cooler, Oil Lines to Turbo, Seals (common Passat problem too), clean Oil Screen, Oil Pan Gasket, new Rings, and new Valve Cover Gasket.

After work that extensive, we received the car back (3 WEEKS!). About 30 minutes after we received the car, we discovered an intermittent problem with reverse. It appeared to be electrical in nature. Returned to the dealer with this problem and, after having the car 2 MORE days, they have identified it as a Multifunction Switch on the transmission (I assume they really mean the transaxle). They are now trying to tell us this is a problem unrelated to the service just completed. I contend they probably messed it up. Right now, we are between push and shove.

Can anyone shed light on whether the error could have been caused when they worked on the engine and if there might not be some other "upstream" problem that can affect the MFS?

Thanks!

New information from the dealer: The service manager indicates that the MFS is "corroded" and that water has migrated into the switch through "capillary" action inside the wiring. Apparently, the water came into the wire through some wiring harness (they are searching back up the wiring for the root) and migrated down into the multifunction switch. Somehow, my confidence in their diagnosis is thin but how do you dispute something so obvious as this? How long does it take for corrosion to form in one of these things?

problaby 6-12 months. On passats, under the battery, there is drain holes for water buildup. When leaves and other related stuff clogs up those holes, water then drains through a relief hole 8" above the bottom that drains into the car-how smart.... But also on the header core side, when the drain hose for the heater core tray and evaporator tray clogs up, water drains into the cabin. Also, water can drain into cabin from the dust/pollen filter.