MKIV Problem: "Ant Open"

[herrubermensch]

I'm an interloper from the M5Board.com with a 2001 E39 M5 with an MKIV unit that I installed about six months ago and that I bought from a seller on eBay. Worked beautifully and seamlessly until I removed it to install a new stereo. It was out of the car for six weeks. I reinstalled it, and everything seemed to work fine at first, but the NAV slowly but surely was "off" a road or two from where it should have been. A week later, it was a couple of cities wrong--about 300 miles off. I went into service mode, and the almanac has not been downloaded and no reception from satellites and the message "ant open." Now, I know from this board that that means a problem with the antenna or the connecting cable, so I replace both. Same problem.

Is there anything I can do here short of replacing the entire MKIV unit? Any "reset" technique that might cause it to recognize the antenna and download the almanac? Anything else to try before I blow $6-700 on another MKIV unit off eBay? I appreciate any help you can provide. Thanks.

--Peter