Almost at the end of my rope with the nav system in my X5.

I THINK it's a MkIII system I've got, but how do I tell ?

Anyway, the CD unit in the back has power in it. It will accept a disc, and I can hear it buzzing away as it reads it. But then it just spits it back out after 15 or 20 seconds, and sucks it back in again. This cycle repeats on and on ....

And the nav system isn't appearing at all on the monitor inside the car.

So it seems to have power and I've checked to make sure all the cables are connected up. But what the hell is going on here ??

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2001 X5 4.4i