I think the problem is displaying three functions - tail light, brake, turn signal - with two filaments.
You can make this work with creative use of some relays, so maybe early VW's did that? Our Speedsters have three filaments so don't need to bother with this. The Spyder, I assume, has only two filaments.
Daytime, all is simple and cool. One bulb for brake, one for turn. Ah, but then night falls and we need a running light. Use the same filament for brake as during the day, but light up the other filament for use as a running light.
But how do you then signal a turn?
Well, when the running lights are on, the blips of power from the flasher circuit are now used to interrupt the tail light (and the brake light, too, if your foot's on the brake).
One of the relays connects the ground of the light fixture to the car's ground. During the day, that relay is unpowered so the light fixture is always grounded. But with the running lights on, the pulses of power from the flasher are now switched to the relay that grounds the light fixture. When the flasher flashes 'on', the tail light (and brake light on that side) lose their ground momentarily and go off.
I think you need three relays to make that all work for both sides. There are probably other ways to make it happen, too, but I'd need a lot more coffee to figure them out.
And something makes me think you might need a judiciously placed diode or two to keep some things from lighting up when you don't want them to.