Here's how to open the doors: Go to Settings > Assetto Corsa > Controls > Patch and they are listed under Extra Option A, B, C and D. Assign controls to those inputs. I have mine set to Numpad 7, 8, 9 and 5 respectively. Hope this helps. I've tested it and it works. If it doesn't work for you, I would check your settings and make sure NumLock is activated.
Thanks a lot. FYI, I just found a better version of this car available on YouTube which you can use some of these CSP additions on if you copy and paste them in.