1999 A4 rear defroster question

OK, after another frosty morning, I decided to tackle my broken rear defroster. There's power at the fuse panel and the yellow indicator comes on when I press the defrost button. So I popped off the C pillar
trim and see a strange translucent blue box. On one end is a white incoming power (I'm assuming it's power) lead and a black antenna lead (with a little black pigtail going to one of the defrost element tabs). On the other end of this box is a black and red lead. The box itself is grounded to the body with 2 sheet metal screws.
black (to defrost tab #1)----\ ---defrost tab #2 \ |------------------| black antenna lead--------------| translucent blue |--------red wire white lead----------------------| box | |__________________|--------black wire
The other tab for the defrost element is missing. Am I correct in assuming the black lead is ground and the red lead goes to the missing defrost element tab?
Any suggestions would be greatly appreciated.
Cheers,
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following my heated wing mirror saga and wire tracking I came to the conclusion that there must be a relay aft of the cars main wiring, that controls the rear screen heater element. (1997/98 A4 UK model)
what you describe, could this be a relay (the relay in question)
white input = 12v 10A red input = 12v signal low power 3/5A (relay coil) black = radio antenna (top 3 wires of heater element)
as for the red and black wires on output which looks the thicker.....this should be the heater element at 10/15A rating. The heater fuse in the fuse box being 10A
one of these is probably ground for the relay coil and the other (thicker) power to the screen element
only thing is I am not sure why radio antenna would go through this junction
clanger
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junction
Clanger is right. I haven't checked yet, but there must be some sort of timer relay for the rear defroster, it might even be built-in, or right behind the switch.
Romy
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I am not clear on the C pillar......do you mean the rear quarter pillar near back window.
clanger
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The rear in-window AM antenna is AMPed in C pillar. Not sure but the defrost might be in the same component but seems unlikely.
Clanger wrote:

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