fully charged battery will drain dead overnight.

You would need to do a amp draw test to determine which circuit is the problem. A stuck relay could cause it but so could a short or something as simple as a switch not shutting off. To diagnosis it your self you would need a low amp probe and wiring diagrams. You would then need to test 1 circuit at a time or each night pull 1 fuse at a time each night untill there is no dead battery in the morning. Normaly we can find the problem in a hour or two using your diagnosis tools. Hope this help. Chris David

Heres a great way to check. Disconnect the positive battery cable and put a test light between the +post on the battery and the +cable. It should dimly light up & even have a slight pulse. If its bright there is a current draw, leave the light hooked up and start pulling fuses one at a time until the light dims and that will be the live circuit. Just put each fuse back in after you pull them. Ive seen alternators go rogue and cause the battery to be drained. FYI I own a 300m as well.