NO... You can not install a nav system on a car with out it. On an IS these are what you need to replace:
nav ecu about $1,000.00
nav antena
nav screen w/motors about $3,000.00
nav switchs
dash wire harness
right cabin harness
trunck harness
dash tray
center counsel
You get the idea...
Prices are not exact
I know this because I am a lexus technician. I would get this question all the the time. Also Is300 w navi come with heated seats and lether interior.
Any questions let me know