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Bill;
Calculate the value of resistance you need using Ohms law.
R=V/I ,(where Ris resistance in ohms, V is normal nominal operating Voltage of 6, and I is the current draw measured). Important: Power dissapated in the resistance MUST also be calculated, and the resistor appropriately sized to handle the load or else it will burn up. Calculate Power rating of resistor using another version of Ohms laws. P=I(squared)R, using the numbers you have, and have previously calculated.
Using dropping resistors is a terribly inefficient way of dropping the voltage to a load (mount resistors on undercarriage sheetmetal to help dissapate their heat). It works by actually wasting an equal amount of power that the load requires in heat...so you'll need that alt upgrade to keep up with the waste...but I recommend using a "two-wire setup which keeps the function of the AMP light. See: http://www.intelab.com/swem/altkit.htm
Another trick for splitting the voltage is to run two loads in series, allowing them to split the voltage between them, but this technique may ONLY be used if the two loads are the same (equal current draw means they will share the voltage equally).
Your best bet is to replace as many 6V loads as you can with 12V , and use dropping resistors on those which you absolutely must keep (gas guage maybe).
I have stock and sources for the resistors you'll need.
Cheers
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