2000 cadillac deville neutral saftey switch location and how to replace

Hi Andre , I'm guessing your Caddy won't start because of your wanting to replace the neutral safety switch .Before replacing or trying to,did check for B+ voltage coming through the switch to the PCM ? There are a lot thing's that could cause your caddy not to start !
Moving the ignition switch to the CRANK position sends a 12 V signal to the PCM ignition crank request input. The PCM verifies that it has received a valid theft password from the vehicle's theft deterrent module and that the transmission is in the PARK or NEUTRAL position. The PCM then grounds the control circuit of the START 1 relay. When the START 1 relay is energized it completes the B+ voltage circuit to the starter solenoid S terminal.
There could be a anti-theft system problem , is the security light lit on the instrument cluster ? Check the start 1 relay in the under hood fuse box . Did you test for Power an control at the relay ? Find the relay an pull it out of the fuse box , look on the bottom of it . You should see four set's of numbers 30,87 an 85,86 , 87 should B+ voltage ,30 goes to the starter solenoid .If you jumped those two the engine should crank over . If the starter is good an battery cables are good etc... I'm talking about jumping where the relay plugs into , not on the relay . Pin 86 should have B+ voltage with the key on , pin 85 goes to the PCM . PCM gives a ground to pin 85 energizing the relay sending B+ voltage to the starter .
Do you know where the PCM is located ?
Transaxle Range Switch Parity Input
connector 1 blue pin 41 ,white wire

Yes you need to test first because what you call neutral safety switch is really called
Transmission Internal Mode Switch
The transmission internal mode switch (IMS) is a sliding contact switch attached to the lower control valve body, within the transmission. The manual valve link assembly then connects the IMS to the manual valve. The four inputs to the PCM from the IMS indicate which position is selected by the transmission selector lever. This information is used for engine controls as well as determining the transmission shift patterns. The input voltage level at the PCM is high (ignition voltage) when the IMS is open and low when the switch is closed to ground. The state of each input is available for display on the scan tool. The four input parameters represented are Mode A , Mode B, Mode C and Mode P (Parity).
If the PCM detects an IMS transitional state between NEUTRAL and D4 while the vehicle is in motion, then DTC P1823 sets. DTC P1823 is a type B DTC.