The bigger the wires...
... the bigger the sparks and the larger the amount of smoke that will be released when you short something out.
It sounds like you and your buddy neglected to install a fuse at the point where you added in your wire(s). If so you're lucky that the wires burned up to protect the rest of the Durango. Using a larger wire in this case would have spelled disaster. I've had to rebuild the electrical harness on three friend's cars and it's no picnic. But I digress...
The wire size needs to be appropriate to what you are trying to power. The clue should be in the wire that is already attached to the "hungriest" device you are hooking up. As an example the scanner is going to have pretty small gauge power wire as it doesn't draw much current, but strobes might be a few Amps apiece so their wires will be larger.
Use this as a general guide: For power runs to a single device, use the same gauge of wires for runs up to 10 feet, and go up one wire gauge for every additional 10 feet or fraction thereof. For example, if you have a 22 foot run and the device has #18 wire, ideally you should be using #14 (the gauge changes by 2 per size step, and a smaller number is a larger wire). For each device of similar power added, increase the wire size one step.
Oh, and don't forget fuses. Again you always must use one at the spot where you are tapping in to the power, and you should also use one for each device. The fuse needs to be sized appropriately, so don't use a 60 Amp fuse to protect fours strobes and a scanner because you have no fuse in the circuit in that case. Find out how much current (how many Amps) each device needs, add them up, multiply that by 1.3, and go to the next size fuse. For example, four strobes at 3 Amps each + scanner at 1 Amp = 13 Amps X 1.3 = 17.9 Amps, so use a 20 Amp fuse.