In most of the MB windshield fluid reservoirs, the top half of the reservoir is able to contain twice as much water as the bottom half (the top is wider as the bottom). That is probably the reason why the warning light gets illuminated when the reservoir is half empty (in models with a straight shaped reservoir).

greetingz,

__________________
1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)