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.
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.