C280 won't start even with new optima batt

Something seems to be drainning my batt.Car will only start when boosted. I don't think it's the alternator because the car will stay running and not stall out. I had a optima batt put in and that also gets drained.I reasently got my deck removed and nothing put back in. That's the only thing I did before this started happenning.I don't want to take it to MB dealership hear cause they are idiots.