For ease of lifting the carpet the left side underseat is the place, BUT then the speed pulse has to run all the way round to the other side.
So then you could fit it under the right seat, you cant lift the carpet as high but you will still get the box in. Then get the speed pulse easy & you can actually get the reverse wire on the right side too
BEware - dont switch the key on whilst the seat is disconnected - so reconnect before you try to test your new nav.
If you are fitting the hideaway under the carpet under seat then remove the foam from the underside of the carpet so the box can breath & doesnt overheat.
Oh and yes the gps can go under the dashtop