A resistor will not help you because its voltage drop is dependent on current. Say you size the resistor to give 8V when the starter is cranking. When you first connect it, the voltage drop will be very low because only the instruments will be drawing power (before they are fried by the 12V battery). If you size the resistor to give 8V with just the instrument load, your voltage will drop to essentially zero when you go to start the truck. You need to jump the truck from an existing 8V battery. Yes, there are voltage converters that can create a stable 8V supply from a 12V input but at the power level of a starter motor they are cost prohibitive.
vir
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