Voltage should have dropped like a rock
Solenoid could bad but if the glow plugs ( you have glow plugs instead of a grid heater?) kicked in the voltage should have dropped
That sounds like the switch or an open between the switch and the relay
Verify voltage at the switch and that it is sending the 12v's to the relay
Do you hear the starter solenoid clicking for sure?
That makes a big difference where the issue is
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