what works for me is to have the dial on vent or floor. not just VW but all my cars the A/C operates anytime the selector is on defrost. not just full defrost but even just slightly over the floor selector to mix a little defrost. also keep the temp or fan dial off any auto setting. Its set up that way to help dry the air to keep window clear in winter and also so the compressor doesnt sit for 6 months not turning. Most cars it wont even tell you its running on a low setting. If your sitting still at idle you can hear the compressor cycle on as you move your selector dial to defrost from floor setting.