could be a wire disconnected?
I agree- running a new sub woofer in the car would mean a dedicated power line from the battery and in line fuse- I remember having one when I was younger (boy racer days!) plus hard wire connection to the stereo system. Given that I read hardwiring a dash cam is prohibited, I think you would be in breach of your contract with Motability unless you get their agreement but I struggle to see them agreeing to it. Ive read that you cant even ceramic coat a motability car and they don’t appear to move on their ruling.
I don’t understand why you would need a dedicated 12v powerline to run a new sub. especially when the car is equiped with a dedicated Amp, which has all the wiring you will need.
Checking the wiring is relaively simple, with a little knwledge of Ohms law.