if it has popped off in the bootlid, personally i wouldnt bother refitting it until all this cold weather has gone because all thats going to happen is that it will just come off again whe the lines freeze up, first hand experience, i'm now leaving mine until the spring and just making do with no rear washer.
Everytime i take off that trim i seem to break another clip off and i'm running out of ones to hold the thing on