I've got a Euro lense with a crack (with a gap) in the reverse part of the lense (clear part). The rest of the lense is in good condition. Has anyone successfully transplanted that part from another tail lamp?

Also, I have a hair line crack in one that I plan to inject some solvent to bond it. Is the syringe method the best?

The lenses are made up of 4 sections. The curved part, center section (two parts actually) and the reverse section. These are glued together to make the whole lens. It should be possible to cut with a thin blade with fine teeth thru the sections at where they join and reglue them together to make a good lens. You will have to be carefull at the outside edges when cutting as these overlap into the next lens. There is little keyways that overlap. Also when you reglue them, stand them up so any glue drips down to the inside and not on to the face of the lens.

Yes. I've done it. You can cut the sections off with a fine tooth bandsaw blade or a coping saw or even a good carpenters handsaw and glue them back together.