The glow plug relay is not your issue.

To find the drain proceed as follows:

Disconnect the negative battery cable and put an ammeter in series between the cable end and the battery negative post.

The reading should be about 50-60 ma. But, it will be higher than this level.

Then, start pulling fuses until you find the one fuse where the reading drops down below 50-60 ma. That's the culprit circuit. Now comes the more difficult part of finding which device on that circuit is leaking current to ground.

The confirmation of a leak above 100mA is the first step in the process.