Hello Mondeo (MK4 2.0 TDCi) enthusiasts.
I am trying to troubleshoot the following faults both of which occurred, became apparent same time, same day.
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Recently this problem occurred at the same time/day on my Mondeo 2010 2.0 MK4 TDCi (hatchback)
The car horn stopped working and at the same time the car sidelights came on (stayed on) until engine was stopped and ignition key removed. Whilst the engine is running then the sidelights will not switch off and neither when switching lights mode to headlights/full beam/spots by using the switch/dial control on dash it does not change the 'sidelights' status.

So horn (doesn't work) and sidelights just come on/ stay on when car is started/running. 
Can anyone advise please? I am assuming that the two faults may somehow be connected or maybe it's just an unlucky coincidence. I have not attempted to look for or change any fuses or relays as yet.

It had been suggested that it may well be a 'relay' problem (no idea whereabouts this is) and even if this the cause.
There are no other electrical faults evident as all other electric items work fine. 

i don't know about mondeo, on 2008 focus the fuses and relays are beneath the glovebox. The big white plastic thing has loads of wiring connectors on it and fuses, but not so obvious is that inside it is an electrical circuit board with computer chips on . this is the BCM body control module which controls the lights etc. 

I am assuming the mondeo has something similar somewhere

I am guessing a bit but I might be tempted to disconnect the car battery for half hour to see if it sort of reboots the body control module (which is sort of computerised).

If your stereo is the type that has a security code you will need to enter this afterwards to get stereo working again. 

Check your engine bay fuse box for water ingress and corrosion especially underneath it.