Here's how I read your post as I haven't looked at a diagram as I'm on my phone
Pin6 at the relay NBV?
Pin4 ECU control (switches to - when glow plug activated)?
So with glow plugs not activated you should have NBV on pin4 and then when activated this should go - bringing in the glow plugs in?
So making an open on pin4 of the relay at the ECU and putting 1 side of the bulb to + and the other side to to the control wire you should have + at the ECU with glow plugs off then when activated bulb should light and 0v should be ECU side of the bulb?
Hope this makes sense as I read your post as if your control wire is shorting to ground?
Regards
Carl