Preheating system diesel van not working

vauxhall corsa combo 2001. no live to glow plugs, changed relay. still won't start. no light comes on on dashboard even if warm
starts eventually when cold, runs ok then. starts perfect when warm.
where is the fuse, found the fuse next to the relay, changed it.still same problem. Surely the preheat light on dashboard should come on when ignition on and goes out immediately if not needed. but doesn't come on at all. changed control unit, same problem, can't find anymore hidden fuses. Ignition on this morning very cold start, no preheat light on dashboard, started after about 5 mins turning over, good thing battery is ok, once started runs great. think the glow plugs now, but why no live feed to them when turn the key? annoyed now
Asked Jan 13, 2016
Edited Jan 15, 2016

TIP: If it's not your answer to this question, please click "Leave a Comment" button under the question to communicate with the question owner.