GPR and or GP
I agree with Bushpilot.
When you turn on the ignition, does the battery gauge drop, then rise up after it runs a bit?
If not, then it may be the Glow Plug Relay not fireing the Glow Plugs (easier to check and replace). Can also check with a meter across the terminals and have someone turn on the ingition. It should allow current to go through, then shut off.
The glow plugs can be checked through the valve cover harness and a ohm meter. Off the top of my head, I do not remember what the good values are, but it does help troubleshoot which ones are TU.
Ron