Unexplained zero alpha in rendering

Hi,

When rendering my F50, whatever appears to be behind the rear window, gets an alpha of 0, while it is clear the back of the car should be visible. The result can be seen in the attached image. I replaced the transparancy with green, to clarify.

At first, I thought it had to do with ray depths, but increasing them all made no difference.

Anybody got an idea why this happens? It’s blenders internal renderer.

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