Hello,

I have Fiesta MK7 from 2010.

My issues has started with rear window wiper and heating. I've checked servo - it works just fine, it doesn't have enough torque to move the wiper on the window. Next thing was the trunk unlocking button. 

Last issue is the top level brake light - LED works (checked with AA batteries and step up converter), I've checked output on the car's wiring - it's 10.5V, but LED works with voltage above 12 (marked as 13.5V).

I guess it's a problem with wiring, how can I fix it?

The usual failure point is the loom that passes between the roof and bootlid.
Constant bending of the rubber tube and the tendency for the wiring insulation to become brittle over time causes the insulation to split and damage the wire.
A bit of water or suchlike causes a short circuit which results in the symptoms you describe.
You need to expose the loom at the tube and check for breaks / discolouration / water ingress.
I had this on a mk1 focus whereby pressing the brake pedal caused the boot to unlock and the wiper to jump.
It also burnt out the boot lock solenoid quite quickly.
Suitable gauge wire and crimps / soldering to patch the wires will do the job.
Make sure you cut the existing wiring back to remove anything that appears discoloured or corroded.

Sent from my SM-G965F (S9+)

Thanks for your quick reply! I've forgotten to mention that I checked that painpoint already, but it seems to be just fine.

Curious.
May be an idea to get an auto electrician to take a look.
May also be worth checking relevant earthing points for corrosion etc.

Sent from my SM-G965F (S9+)

That's why I tried to grease hinges (I thought it's not grounded properyl), but TBH I have no idea how to check if bootlid is grounded properly

Locate ground wire from boot electrics (e.g. brake light) and use a multimeter srt to continuity.
One probe to earth at light and other probe to bare metal in the boot (rear seat locking loop is good).
If the meter shows a zero then the continuity is good, if you get a 1 then the circuit is open (something broken).

Sent from my SM-G965F (S9+)