So we have two batteries. I would "assume" one is for the glow plug and the other is for everything else. My questions is which battery is better used for connecting after market electrical equipment to?

 

The two are hooked up in parallel. The starter draws allot of current when starting the engine due to the high compression ratio of the engine. Were a car would draw 200-300 amps when starting the diesel draws 500-600 amps @ 12 volts. You should have a 130 amp alt. or larger on your truck. I see no problem hooking up something else as long as it does not draw over 40 amps. All the electronics along with everything else in your truck needs around 80 amps.

 

Batteries are wired basically to make them one big battery. I prefer to use the driverside bat because my exhaust and airfilter are on the passenger side. Makes it easier to run wires.