i have an 83' 300d. glow plug light on dash does not come on when i turn the key on. does this mean the glow plugs are not working? car runs fine once started. starts fine in the morning after being plugged in all night. hard starting in the evening after being in the exposed work parking lot all day. i noticed the glow plug light did not come on one day after work. got it started and drove home. turned it off and back on and dash indicator worked. next morning nothing. please help