Where are the PCM and REM located on a 2002 Jaguar S-type 3.0 and how does one access them?

you definitely need an ignition coil or two.
s types need coils regularly.

Check fuse F2.7 15amp located in the (CJB) central junction box , this is the fuse box inside the vehicle . This fuse supply's B+ voltage directly to the back up lights . Now is were it gets complicated , the lights are connected to the (REM) rear electronic module ,which will supply a ground when commanded too. The (REM) is connected to the SCP - it's a serial data network that all the electronic modules on your vehicle talk an share info. The PCM - engine computer receives inputs from the transmission range sensor switch as to what gear the vehicle is in . This info is sent out on the data bus so which ever module uses it can . When you put the vehicle in reverse the PCM sees this an sends a message to the (REM) to turn on the back up lights !
010 Ford SCP Communications

ICM - PCM - ECM on the web:
images for 2002 jaguar s-type 4.0 PCM location

do u mean the powertrain control module? if so it is located on the l/h side under the cabin filter housing. remove wiper arms, windscreen plastic scuttle panel, metal brace bar, caution. the 4 outer bolts can break off easily. and cabin filter housing , the module is held with 2 nuts remove connectors and nuts the module then comes out from inside the car remove footwell liner for access

may i ask what type of s type you have 3.0?4.0? or 4.2?

There are no fuel pump "relay" fuses. There is a fuel pump fuse that protects the secondary (contact) side of the fuel pump relay, which supplies power to the Rear Electronic Module (REM), which in turn supplies power via a pulse-width generator to the fuel pump unit. The power for the primary side of the fuel pump relay is supplied by the REM and is protected by the fuse that protects the REM itself. The REM has 3 separate fused inputs and the manufacturer does not provide information to indicate which of these fuses provide the output power from the REM to the fuel pump relay.

If you are trying to diagnose your fuel pump circuits, it is necessary to use a scan tool that has vehicle-specific capabilities and can access fault codes and live data from both the the Powertrain Control Module (PCM) and the REM. Two-way communications and actuator testing is also necessary for certain tests involved in verifying the fuel pump circuits.

The easiest way to verify whether the entire system is working or not would be to check for the proper voltage (over 12.5 volts) at the Fuel Pump Module (FPM) electrical connector. If the correct voltage is present at the FPM connectors, then the fuel pump relay, the REM, and all related fuses are good. However, to do this, once again, it requires a scan tool as described above so that the fuel pump can be commanded ON through the computer system functional tests.

CHECK IGNITION AND PCM FUSE.YOU COULD HAVE SHORT OUT PCM. IT CONTROL FUEL PUMP. CHECK FUEL PUMP RELAY.SCAN VECHICLE FOR PCM FAULT TO KEEP FROM SPENDING A WHOLE LOT OF MONEY.

Its located at the front center of the engine (it have Jaguar logo), you need to remove 3 screws of the large panel and the oil cap, remove this large panel and then remove other 6 screws of the air filter box.

most often the access point is in the trunk. there will be a removable panel.