My 300SD does not light the glow plugs indicator, does it mean I should change glow plugs because I am down to one?

It starts fine but now at around 12-15F it got harder indeed, few more cranks and rough few seconds after starts, then like a dream

http://www.peachparts.com/shopforum/vbpgimage.php?do=full&p=3664&d=1229373678


BTW, BOSCH or BERU?