The antenna will only be under the dash if sat nav was retro-fitted. In this case, the BMW instructions say to put it on top of the air ducting behind the speedo / tachometer. This is because this is the only easy way of getting behind the dash. Actual postion for a retrofit will be different for LHD/RHD X3's. There must be no metal directly above the antenna - plastic and glass are ok.

I think factory supplied navs put the antenna in the shark fin on the roof:

See item 8:

http://www.realoem.com/bmw/showparts...94&hg=65&fg=22

I'd guess that your antenna is in the sharkfin - these are hard to get to since you need to remove the roof lining as the fin is glued and sealed.

You'll need a 5 metre antenna if you decide to add a replacement behind the dash. Routing the cable through dash, glovebox and down near side sills is reasonably easy but involves removing lots of components in a UK RHD X3 - see my other posts on retro-fitting sat nav to X3.

The following diagram is from the sat nav retrofit instructions for LHD X3's:

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