I have a 1990 M104 300CE and I didn't need to mess with the back seat, the fuel tank is in the trunk just look all the way forward to that panel and pull on it, it will come loose, can't remember if you pull it up or what but just pull on it, once out you can see the fuel tank, the sender unit is at the center of the tank facing rearward near the top, but not on the top. Has wires going to it. Could be they are not all the same, but that is how mine is...here is a photo of it. To replace you have to unscrew the big nut, it's really big, I used a pair of large channelocks pliers then the sender pulls up and out, with a little fiddling.