Not happy with car tracker fitting.

On the 11 January 2021, the tracker device was installed in my polo vivo, and it was not the first time I am doing business with them but this time I am really disappointed with them. What happened is that that day I left the car with their technician/ installer and I signed the installation/ insurance certificate document before the guy put the tracker because I trust them as they are professional and left to work as it was not the 1st time installing tracker with them, and I told him that I will find my car keys on my father in law work place when has done the fitment as it was the secure place for him to fit the tracker, I left the car with no air bag light on. When he has done with the car, he then left everything with my father in law even the proof of the fitting of the tracker. But te funny thing is that he wrote that he find the car with an a bag light on, and I have already signed the installation document before he installed the tracker and I trusted and He didn't gave me installation certificate but he left it with my father in law, but then he didn't bring it to me because he knows that when I signed it before my airgabag light was not showing on a dashboard but he made it looks like was at work and I trusted him , and he wrote that on the note that I got from my father in law. But I then called the call center the following day as it was late to call that day, I explained to them that the was no an airbag light showing on the dashboard before the tracker was installed and I told them that maybe the guy saw that he damaged the wire or what ever that causes the airbag light on, it was going to be easy for him to write that on the installation feedback as I have already signed the fitment document before he touched the car. The call center told me they going to book a call for their senior technician to come and have a look what really happened, I been following it up for a week till he came on saterday 23rd, the he checked the car. He come back to me at work with a feedback, that he checked the car and the was no fault on their side, and he can't read the computer box of my car, that the computer box is not for that car, and I have an alarm system on the car myb it is cause of that, and he can't read clear any errors due to my computer box is not for my car and so on and so on and my head lights are not working, they have been breached. But what is that got to do with installation of a tracker. He then told me he requested the copy of the fitment from the guy who installed the tracker, he showed me that have signed and agreed that the Airbag light was on before the tracker was fitted. I then told him no they guy is lying he did wrote that after he realise that he damaged it because a already signed the document before he fit the tracker it makes look as if I have agreed that the airbag light was always on the dash board all along. He told me no it's not their fault its my responsibility and I can take the car for inspection to VW. According to him it was OK for their technician to lie and put it on the report that He found the Airbag light on.

I really don't know what do now can you please help to fix this, should I go legal way or what.