Interesting information about the parallel resistor for the bulb in the field circuit being a standard fitment on some trucks. What I think I'll try is a 24 Ohm resistor in parallel with a 12V LED in the dash, this should mimic the resistive loading of a 2W bulb (with a stone cold filament) in the circuit, hopefully the current split through the parallel resistor and LED will be sufficient to light up the LED.

Thanks to all. Stuart.

1950 Chevrolet 3100 (occasionally working) truck:https://c2.staticflickr.com/6/5490/30684982201_6dc479d18d_c.jpg
1928 Chevrolet AB National Phaeton: https://c4.staticflickr.com/6/5573/30137792083_42886d30a4_c.jpg
1932 Austin Special Sports: https://c8.staticflickr.com/6/5793/30116567183_1520e67c4d_z.jpg