New battery fitted to 09 fiesta. Now when locked alarm goes off randomly. When moving alarm goes off. Dashboard says boot open. It's not. Please help.

Open or remove side panels in boot ,probably corroded contacts.

Possible the battery voltage is falling too quickly fooling the brain into thinking a door has been opened.

Worn, maladjusted or damaged door, bonnet or boot strikers or worn or damaged locks is another possible cause.

You may have a broken wire in the harness that runs from the car to the tailgate.

The wires bend backwards and forwards every time the tailgate is opened and they eventually split the insulation and the wires poke out and touch each other, causing short circuits and problems with the alarm.

Take it to a local mechanic and get a quote to fix it.

I've done this before and it's about 2 hours work.

The sensor is located and probably built into the boot latch. The sensor is good because the light goes off if you close it slowly. the latch is just worn out with that particular part. Replace the latch. It is not hard to replace if you have a few hand tools.

We tried to open Fiesta as shown in Video...

Replace battery in remote.
Check wires inside of the boot in the door jam (slice hole in boot or pull out of one end. Check wires carefully for bubbles and weak spots from cracking.
Check wires inside door jam or under dash to left hand side carefully with flashlight for bare wires shorting. Feel free to move things around under there a bit to see if things are shorting or what have you.
The fuse box under the hood - inspect underneath of it for corroded wires and or contacts - may take a bit of work to remove it from car.
See if alarm is cleared by using drivers door key as there is a switch in there for the alarm. If this is faulty then this could be your problem beyond the wires in the door jam perhaps.
You have a short or broken wire or this switch in the door is bad.
These are some suggestions of where to start.
Do it in order of easiest but I suggest the door jam wires inside boot is most typical problem especially in colder climates like here in Canada East as the wires freeze and crack after a while.
May or may not be your problem but options.
Let us know with feedback about your progress or solutions plz.

IS THIS A FACTORY ALARM OR AFTERMARKET

Alarm circuitry usually memorises the battery voltage half a minute-ish after being set. If the sensed battery voltage falls too fast because the battery is (almost) faulty or in a low state of charge, the alarm brain will believe a door has been opened and switched on the interior light.
That is one scenario of several possibles...

Shaking the car is very unlikely to trigger a modern car alarm though opening a door should - because it doesn't it might indicate it isn't being armed (or fully armed), perhaps because it can't find a stable battery voltage to lock onto or perhaps it thinks there is a door, bonnet or boot open due to defective switching, the bonnet is most likely.

Over a period of time most cars will drain battery. Clocks ,alarms and other bells and whistles that the new car have all need power.

i have seen a low battery on remote cause this...how old is battery in remote