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Did you measure voltage drop across the pump while it's trying to start up? The voltage will only drop when there's current being drawn. So with the pump off you would see full supply volts even on a highly resistive line. You could try lugging a battery down to the pump, and jump it to get it started, then remove the battery jumpers but keep the pump running on your main run, then measure the volts across the pump.
Or you could try to measure the resistance of your wiring with your meter. You would disconnect it from your 12v source, then short the pump side, then measure the ohms across the pair at the source side. Also measure the resistance across your pump windings when it is disconnected, but with the switch on.
I'll throw out some example numbers. Say you have a pump that says it needs 6 Amps. Chances are the inrush current is more like 12A. If you measure across the terminals with the pump disconnected from power with the switch closed, you might measure around 1 Ohm. Now if you measure your very long supply line and get 2 Ohms. Then you would only get 12V/(1+2 )Ohms = 4 Amps through the whole system. With the pump drawing 4 Amps you would only see 4V at the pump terminals when it is already running.