Did it spark like it was just sucking a little power to charge a rogue capacitor, or did it spark like there was a short circuit? BIG difference between the two.
A capacitor spark will be a small short over and done with, not to happen again because the offending capacitor is now charged again, or possibly like a slightly newer radio was installed and the memory capacitor is sucking power, or maybe even a newer vehicle with all kinds of electronics trying to grab a little part of that spark, but just for a part of a second?
Is it like a spark like touching the wrong two wires together when hooking up a new trailer wire harness? Nice, long, and heat your fingertips right up.