It sounds like there’s a short at the 12v power input somewhere.
Consider Ohm’s Law: V=IR. Voltage equals current times resistance.
The battery wants to be V=12 volts between terminals. But the resistance, R, of just the wires and their contact point is so small, that the current, I, has to be absolutely effin’ huge.
Now, consider Watt’s Law, P=IV. Power equals current times voltage.
Since V is… about… the same as it usually is, and I is “absolutely effin’ huge,” then their product, Power, is also going to be “absolutely effin’ huge.”
Since Power is defined as rate of Energy consumption or supply, this means that the amount of electrical energy being deposited in the resistance, i.e., the wires, is “absolutely effin’ huge.”
And finally, since electrical energy deposited in resistors gets converted to thermal energy, aka Heat, then the temperature of the wire is gonna get “absolutely effin’ huge.”
And that’s when the insulation melts.
I would check to make sure you didnt short the power wires at the input, and that there’s no short inside the unit. Since you just had it serviced, the latter is entirely possible.