It can be accessed from the trunk (or cargo area of a 245). After removing the carpet, you'll find a steel plate with wires running into it. Remove the plate and you're looking at the top of the tank. There's a "bung" (plug) which holds the assembly in the tank. You need a special spanner wrench to remove the nut properly, but it can be done with a hammer/drift or two screwdrivers crossed.

You do not want to do this job as preventative maintenance. It is a miserable job. Test to see if your pump is working first (various methods have been devised, search the forum).