Me and my dad are trying to restore a series 3 109, ex-Dutch military 24v. When we bought it, it was running, not great but running, a lot needs to be done, we did the break but now the car won't start and it's causing the battery to spark. I tested it and I got a short when turning the key. I'm not an electrician nor a mechanic, I tested to see if any other button of the dashboard was when turning the center knob, which I think is related to light, and when turning the center knob, I tried to see if it was related to the fuse box, but no, I still get the short even though all the fuses were removed. I have no idea where to start diagnosing.
Any help would be appreciated. Thanks

Get a multi meter. Then you can start looking for the problems. First. You do have 2 batteries, right?

I do have 2 new batteries and a multi meter but I'm lost to where to test, i already tried on some of the electrical but noting cause a short