I am sure there is constant 12V on the line, though the empeg stops immediately; led goes out and harddisk stops abruptly

Well, that's what's causing the thump. It should just go into standby mode at that point, the hard disk should spin down gracefully, etc.

How about when you put the empeg into the sled while the engine is off; should the empeg power on?

Not so sure about that one, I don't own a Mark 2.

I tested the direct 12V line this time, but with no result...

Blown fuse on the constant 12v line, perhaps? Triple-check your wiring and fuses. Look in the Empeg's owner's manual for the color-coding on the hookup wires, and triple-check that the wires are really correctly hooked up. How sure are you about the car's wiring pinouts?

Make sure to let us know when you resolve this, because I want to put it in the FAQ when we come to a conclusion.

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Tony Fabris