Ok, so here’s what I would do.
Leave the usb connected as is. That should work fine.
I would swap the wires round on the fag lighter so that the red wire goes to the centre and the black wire goes to the body. This means that anything plugged into it should be the correct polarity, but I would test it with something ‘disposable’ first - e.g. an old power supply.
The black wire MUST be fused as close to the flying end of the wire. The fuse in the red wire can be left in place or chopped out. The main bracket looks to be plastic so that is fine.
I can’t see any exposed metal apart from the fag lighter body, but if there is any, either give up on the idea or make absolutely sure that it can’t touch the body of the car (shouldn’t do any damage as the fuse should blow) but best to be sure.
Red supply wire to earth, black supply wire to a live feed.
I know it all looks to be about face but I’m 99% sure that will work for a positive earth vehicle.
Feel free to ask away if you have any questions.
Edit: looking at the pic again, I would wrap insulating tape around the exposed metal parts of the fag lighter outlet just in case as this will now be live. And make sure that anything plugged into it hasn’t got any exposed metal as an ‘earth’
