Ever since new, my 02 echo hatch (aka yaris) has had bad rear suspension. With 4 average size people in the car (150lbs each'ish) the car bottoms out hard over any slight road imperfection

. Not cool...
So, i recall back in the old days, my 1991 dodge daytona shelby had similar issues, and i had some airbag helpers installed. This way it still rode nice when empty, and did not ride "deadbody in trunk" style that it did when i had some cargo in the back.
Long story short, does anyone know of an airbag helper kit or stiff rear springs for the echo's that would help out with my problem?
Please and thank you,
-mark