Got a cry for help from a neighbour - his '93 Passat Estate had a blown stop light and he couldn't get it working. He'd gone to Halfords, looked up the replacement and bought a couple of spares. Except that neither worked. So asked for help.
The reason soon became obvious when I looked. The bulbs Halfords list as replacements were standard 5/21 watt 180 degree offset pin types - the sort that have been around for years. But the Passat used 4/21 watt types with 120 degree offset pins - which I'd never seen before. And 4 of them - all twin filament but with only the one second filament in use for the one stop lamp. So swapping bulbs would have sorted things. He also had tried to fix the nearside rear fog light. No point as it wasn't wired. ;-(
Halfords argued about refunding for the wrong ones. They said it was VW's fault. They did have the correct ones in stock but not listed under Passat in their cat. And of course are more expensive than the more common one.
It seems strange that VW should use 4 more expensive bulbs than needed but save on a few feet of wire.