Originally Posted by VUSSUTE

Do a resistance check first mate. If its less than 100 Mohm, check your stereo...Most aftermarket stereo setups tend to have power fed to the stereo via a direct feed from the battery and bypasses the accessories relay which causes the discharge. Any sections of wire that are spliced together, make sure they are heat shrinked or appropriately isolated with electrical tape.
I promise you 80% of the time this tends to be the issue.