I'm so sorry for what you're going through.
This is SO much bigger than an apple airtag, which is just the tip of the iceberg.
You're the victim of an utterly appalling situation.
As a British woman who became Muslim and married a Pakistani family (which is what's happened) you're subject to grinding racism from your husband's mother - 'English brain' is being used as a racist insult - and he does nothing to protect you.
Your appearance, dress, parenting, everything are routinely insulted in front you and the family - and he does not defend you.
And in fact, he sides with her.
Your own mother is driven out in distress and your husband and his mother reject her.
Your children have noticed.
You were left after an awful birth experience to struggle and serve her, with zero practical help - she's only 49 but is playing the poor old lady card.
Your husband sleeps in her bed when he goes back to Pakistan.
She LIED about you and the baggage incident and he chose to believe her.
Your SIL feels close - but never forget she was raised by the same toxic influence.
He has now buggered off and left you to struggle with two children and a new baby with no nearby support in freezing weather, while he lives it up on a long family jolly on the sun.
He and his family are apparently either gloating or utterly oblivious to the effects of their behaviour and sending you all the happy pics of them having a ball.
And he has spent thousands of pounds of YOUR joint money that should used be for your own family on this giant self indulgence which isn't even the actual wedding.
Latest: you've now discovered there is a tracker in your car. This is YOUR car. Despite your best efforts you can't find it - so it hasn't accidently slipped out of the bag, it's been deliberately and resolutely hidden.
As an outsider they don't trust you and MIL has probably poured poison in his ear about you, prompting him to do this. He is that hard combination, both a weak mummy's boy and a misogynist.
Don't play any of the games suggested here, tempting though they might seem. They would gang up and use that against you.
Make a video inside your car, capturing the beeping sound. Screenshot the alert and any maps of your moves. This will be evidence for your peace of mind should there be any attempts at gaslighting and denial.
Contact the police and tell them the truth - he has secretly placed a tracker in your car and is monitoring your movements while you are not under his control. Get a report number.
Maybe take the car to a garage and get them to locate and remove it - get a video of them doing it, if possible.
They are going to gang up on you whatever happens. Right now you need your mum, family, friends around you. Start contacting and explaining what's happening and asking for help and support.
Do all the usual getting ducks in a row stuff, re finance and seeing a solicitor and contacting Women's Aid.
I do hope you'll be ok - I'm not surprised you're feeling so low after everything you've been through.