Astra Diesel Glow Plug Issue

That sounds like a defective glow control unit, yes. Have you checked the glow plugs themselves? A faulty glow plug can cause similar symptoms, and it's worth ruling out before replacing the entire control unit.

Glow plugs are good. Any other ideas about the cable?

If the glow plugs are confirmed good, and you suspect the cable or plug connection, it's best to have a workshop perform a diagnostic test. They can properly check the wiring harness and control unit. It does sound like a plausible diagnosis though. I had a similar issue on my old Vectra years ago.

Thanks. It was the cable; 95€ at the shop. Sorted.