If you can find the connectors - back to the battery, try cleaning them up.
Where your battery is honestly beyond it's sell by date .. replace.
If neither of these solve you problem, then run a brand new 6mm2 cable (fused) from the battery to a heavy duty (40A) car relay, just by the glow plug. Use the glow plug wire to operate the relay ONLY. Take the switched 12V from the relay to the glow plug. You should find the performance of the Eber is transformed. Hopefully.
It sorted my heater out anyway