I have a rear tank in my 300tdi
Got it fitted in the UK before the car was shipped to Australia. It fills up from a y peace on the original filler neck so you can fill the back normal tank at the same time. Then when the main tank is empty you flick a switch and a pump , pumps the fuel from the rear tank to the front tank. Used this for our 25,000 mile trip around Australia
Here is the tank under the back of the 90, the tank is stainless so should be ok for a while
And the filler next for the tank
Been on the car nearly 6 years now and still use it most of the time
This is what the rest of the car looks like
And as you can see or can't you can't see the tank form normal angles and there is no extra fuel filler
Ali