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You might want to have a little more details for us to look at. etc. what kind of GPS? Factory? Aftermarket?
If you bought a factory one from another LS400, you may be able to get it to work. I'm not sure but someone will chime in on that, if thats the case. IMO an aftermarket headunit with built in GPS might be the way to go.
factory GPS requires a lot of harness and several sensors to work properly and is HORRIBLY out of date. (last update was in 2008 iirc). better off with an aftermarket GPS headunit or a standalone GPS.
As PD mentions, the factory unit is out of date. Your easiest option is an aftermarket unit. Whether it is standalone GPS or integrated with a new head unit would be the current options. The current GPS form Garmin, Magellan, Tom Tom, etc route pretty quickly.
The Garmin mounted on a bracket in my similar 2000 LS400 is wired into the accessory circuit so that it turns on and off automagically. Works great and and I can even send information to it wireless (e.g. from Google, from Yelp) from my Android phone: https://www.clublexus.com/forums/ls4...lan-nokia.html
If the Garmin ever breaks or becomes obsolete, I will just buy another, plug it in and be on my way.
It's an inexpensive solution - the Garmin 2595 is currently less than $199 at Costco and it even has a traffic information receiver and re-routing around traffic jams. Another link in the above thread is about the aux-in port I had added to my radio so I can stream audio to it from my phone -- about $100. The only thing missing is A2DP Bluetooth wireless streaming which I could probably add for another $20 or so.