trying to add Honda sat nav as a fit afterwards would be a complete nightmare
You would need:
New top dash part (with the LCD screen in it)
New Sat Nav DVD player
New dash front (with the jog dial in the middle) - probably
New steering wheel (with the voice buttons on it) - probably
metres of wiring (probably)
GPS mouse (no idea how it connects to the system)
some method of telling the car that you have Sat Nav fitted
If you go for it - big kudos!
