I had this in a 400E for a time. Take readings with a simple digital voltmeter from Radio Shack. Pull fused circuits on at a time . See if there is battery draw hr by hr from the battery. Especially touch your hand to the to the radio amplifliers in the trunk to see if they are warm (battery draw). Disconnect the power to radio antenna as they stick on instead of shutting off. You test these things and you will find the power draw. I bet it is the radio amplifier in the trunk. Good luck. The digital voltmeter shows readings in tenth and hunredths of volt which is what you want. Should cost under $20 and be battery powered.