Well, that link you posted does specifically say, "New Beetle, integration with factory HU, no cutting of harness", so I'd guess it'd work as long as you're still using the factory HU.
I've been looking at this kind of thing to add to my Golf too. I'm happy enough with the stock speakers to not replace them all, but a sub would be nice. Shame I don't have the cash spare.

I was knid of hoping to swap two wires on the wiring harness and plug it in, then handle the RCA to line level thing with an adapter and some creative soldering. Don't care about the sub. Trying to find a quick and dirty way to do it (no room for a double DIN setup, and since the car won't even be registered in my name, I'm not drilling/wiring anything).
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Heather

"I distrust those people who know so well what God wants them to do because I notice it always coincides with their own desires." -Susan B Anthony