Is it in one of the lower corners? Sounds like it is delaminating. This is happening on my 1991 Mercedes rear window. So far still isolated in the corner, but it has enlarged a tiny bit over the years. Also started to corrode the wires from the defroster. Probably no choice but to either live with it or replace it.
glenmore
1991 300CE
1990 LS400
2000 C280
Thank you for the reply anyone else?