The Nissan heater installs in the block near the front exhaust manifold and I think you get access from under the car. It must be one of those 'dealer installed' options.
Be careful though, when I went to order one for my '99 SE I was told that it cannot be installed on '99 Maxima's that have Cali spec since on these cars the first pre-cat exhaust manifold assembly interfers with the block heater wires.
Although I live in New York state, my Max is Cali spec (I have been told that many states in US have required Cali spec for several years now) so I had to give up on the idea. Hopefully Maxima's sold in Canada have no Cali-spec issues?