Having said that I guess that if you can work out the amperage drawn from the section in question by using ohm's law.
Which is V[E]=IXR.
V= Voltage
I= Amperage (current)
R= Resistance.
Here is an example with a 12 volt system and 4ohm speakers
you would be drawing 3 amps (3X4=12), BUT!! there are many reasons that this might not apply to your Truck , your amplifier may have a transformer which draws a higher voltage, your speakers may be (commonly) 4 or 8 ohm AND could be wired series OR parrelel (spl) -which means a different resistance again!! etc,etc,etc.
I have had no problem using these calculations on low amperage active speaker systems (DC) at home, but having had a Yamaha 250 burn it's wiring out underneath me (waaaaaaaaaaaaaaay back) I know auto electrics can be extremely...worrying.
Maybe if you talk to your local garage nicely they'll advise you for free.
Accuracy,you say? hmm interesting concept..
