Citroen Xantia fog and reversing lights not working

left hand side .if this is a RHD vehicle they still fit the lever for a LHD model

Find the central locking box and remove wire connected to Remote alarm box.

try to remove fuse ... and show to car electrition

PSA cars with analog instrument clusters have the STOP light to alert drivers of irregularities in engine temperature or coolant that may cause severe damage to the engine. Check your radiator coolant level and top up when it's low. Don't forget to bleed the air from the cooling system. If problem persist, check for loose connectors to the Coolant Temperature Sensor(mounted on the cylinder head) and the Coolant Level Sensor (mounted on the side of the coolant refill tank). For the alarm, check ALL door switches including those for the front hood and the rear trunk. They may be shorted to earth or one of the door becomes ajar. Next time the alarm triggers, check which door looked like it's slightly opened. If so, realign the door hook located in the door pillar.

The S light is also used like a 'check engine' light for auto transmission. There maybe some error in the transmission control system. You transmission has a computer that controls a lot of valves and solenoids in it. Check your transmission fluid level and it's integrity. If you have not changed the fluid for the last 50000kms then it's due for a change. Use only Peugeot recommended fluid. Other fluids will damage it. Mention your transmission type when buying. It's either AL4 or 4HP20. Best to buy it from authorized dealer. Garages that have a Lexia 3 scanner can scan for error codes for the transmission.

get a mechanic to check your oil and fluid levels and your brakes
its most likely that one of your fluid levels is slightly low

Check on aliexpress for a silca key programmer V33.
This is a programmer for the ECU which you can use to reprogram the ECU to accept ID chip keys. If I remember correctly xantia uses ID46 or ID48 keys. You buy a used key, remove the circuit board and ID chip from it, put it into your own key, and then you have to program all your keys to be valid for the ECU. I haven't tried it on my own xantia, but have used it on another car with succes. It should cost around $80. https://es.aliexpress.com/store/product/New-Auto-Key-Programmer-V33-02-SBB-Key-Programmer-For-Multi-Brands-Brazil-SBB-Silca-V33/2341061_32845994522.html?spm=a219c.search0604.3.2.7e3dcc24iRDWLM&ws_ab_test=searchweb0_0,searchweb201602_6_10065_10068_10547_319_10891_317_10548_10696_10084_453_454_10083_10618_10307_10820_10821_10301_10303_537_536_10059_10884_10887_321_322_10103,searchweb201603_52,ppcSwitch_0&algo_expid=2203bafa-d1d5-4ba3-a1dd-d9f9fd1ed675-0&algo_pvid=2203bafa-d1d5-4ba3-a1dd-d9f9fd1ed675

try changing the relay for the front fogs,the front and rear are not connected in any way,check the fuse box lid for relay location ok

I think there is air trapped in the radiator. Remove the level semsor in on the radiator and make sure you fill the water from there first, fit it back and then remove the expansion tank and hold it higher in the air and make sure youget clear running water at the air bleeders. Cheers.