1992 Nissan 240SX Overnight drain

I have a battery killing drain in my 240. I've had the alternator replaced and have went through at least 3 batteries (thank goodness for Costco & Walmart) When I put my meter between the negetive post and the ground lead I get a -400ma reading that disappears to O when the fusible link to the alternator is disconnected. On the electrical schematic it looks as if the alternator is the only thing on the line and I've already had that replaced. Where do I go from here?