Just get one from Amazon/ebay. My car has sensors, but no camera and the view out the rear window is terrible (Jag XF). I can cope with it, but sometimes I have to let the missus drive it, so I installed a dashcam with rear camera that goes over the rear view mirror. When you go into reverse, it swithes the mirror into a screen for the rear camera. There are cheaper options out there but I find this really good for both a viewer and a dashcam.
Sorry, I thought you were looking for one. I didn't realise you already had one until I read the post again. If they refuse to fit it, tell them you don't want it. The one I posted is a simple fit. The only bit that takes time (probably 20 mins) is trailing the wires around the trim/under the carpets. I hard-wired mine, but you can simply plug it in the ciggy socket
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https://www.amazon.co.uk/Recording-Streaming-170°Wide-Parking-Assistance/dp/B08HCYB17X/ref=sr_1_4?crid=Z6Z3CF9HE1MH&keywords=rear+view+mirror+dash+cam&qid=1706965791&sprefix=rear+view,aps,85&sr=8-4