I have a BMW 316i M40 1989, in an excellent condition. The version with 2000cc and above have a thing called "check control", and it's located right above the rear view mirror. It's a small device that tells the driver if any of the electrical devices such as: front lights, rear lights, foglight, airbag, etc. ain't working properly. My car, however, doesn't have check control, and I thought it might be a good place to put a built-in GPS Navigation. I have managed to find the trim and the sunshades from the model that has check control, and I was pretty lucky to find the navigation which shape and size matches the one of the original device.
I must say I'm not a great fan of customizing, I still have 99% of the original BMW parts, original radio, CD changer, speakers (6 of them, including the twitters in the front doors, which are pretty rare) - everything added later; but, I am not against customizing if it does not stand out too much, and if it matches factory look.
So, here's my mini-project