I had this question today so wanted to relay the information on here. The AIS units on previous models use to be behind the chart table where the main board is located.
However on the Jeanneau 51 Yacht the AIS650 transceiver unit is located under the bed in the port VIP cabin, see photo below
If you lift up the bed section you will find the AIS unit furthest to stern, along with the AIS 100 splitter, VHF Ray 260 control unit and the Raymarine ACU 400
If you are looking for some of the other Raymarine components, please have a look at the schematic below which may assist further. Of course if you have any questions, please get in touch