where is the access point to the fuel level sender unit on a 1990 300 SEL

on top of the fuel tank where you unplug the lead is a big screw. This is the fuel level sender. It's a long aluminium tube inside the tank.

Take out the rear bench seat, undo the 10mm nuts holding the rear of the seat in place and push the back of the seat up to unhook it. There's a big round plastic cover in the rear fire wall, pop that out and the fuel sender's in there.