I would test with a cheap antenna to see if that solves it. You can get a generic GPS antenna with a matching coax plug for under $20. Just temporarily hook that up and drive around for a while. That will tell you whether you have a bad antenna (or antenna cable) or a problem with your MKIV.

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input