Have 2004 Accord 2.2 Diesel European Car. My battery keeps draining overnight. Have noticed that plastic cap from back of low headlight is missing and I am wondering would this be the cause of battery draining due to water getting in etc?

 

I would think that missing cap is NOT causing the battery to go dead. After all, you DO turn off the lights when you park, right?

Got anything that doesn't turn off like it should? Open the rear-seat pass-through & check whether the trunk lamp stay on.

Disconnect the negative battery cable overnight. If it STILL goes dead, then it must be a bad battery.