This is a common failure, although I can't explain why... Hella must have used a poor adhesive to attach the glass lens to the plastic frame. I've had to re-glue both lenses on my one car, and my other car needs the right side glued (left side still OK for now). I used clear RTV silicone and it seems to be working well, although I'm not sure how long the RTV will hold up compared to the original adhesive (hasn't been long enough to tell). I used MB RTV because I had it handy, although I'm sure any similar adhesive would work equally well. Detailed photos of my saga are on my website at
this link.
