I had a VDO-Dayton MS5000 (color screen) on my LT when I lived in Europe, and put it on my US LT when we moved back to the states. It still works just fine.
Check this thread
for more info. There are pics of the various "factory" BMW GPS options in this post
, and links in this post
to the old BMWLT site that give a ton of detail on these units.
BMW mounted the GPS head unit in the left saddlebag, similar to the factory CD changer mounting in the right saddlebag. I made a custom bracket to mount mine on top of the CD changer in the right side on my Welsh LT which took up a lot of room, but left the right bag completely free. On my US bike, I moved the GPS head unit to the factory stereo position above the gas tank.
Note that the GPS head unit reads map CDs, not music CDs, so it doesn't play music at all. So I lost AM/FM/CD, but I gained back both saddlebags and instead I had an iPod for music. I even managed to wire the iPod to use the factory handlebar radio switches so the iPod stayed protected & dry inside the oddments box.
All told, I've put well over 100K miles on this setup. As with all consumer electronics, newer GPS units are much better, cheaper, and faster. But 10 years ago, this system absolutely rocked, and it's still plenty useful even today with updated map sets.
Let me know if you have any specific questions.