Not a source, but context: http://www.hermetics.org/Sabians_of_Harran.html
There are a few papers on the "Sabians" of Harran. I'm not sure if we could say that Harran introduced the Greek world to "Thoth/Idris". Perhaps it came out of Alexandria. However, it was certainly a late Hermetic center, and there is a strong continuity at Harran with ancient cults.
We maybe should be asking this question and finding a better answer. If Iah/Idris were in Syria in the second millenium BC, that would be very interesting. There are Ammonites in Canaan. Sin/Sinai. Etc. Harran was definitely a very ancient site for the Bablyonian Sin.
Also, there does seem to be a difference between Zeus Hammon and Zeus Kaisos. In the Ammonite system, Thoth is a Saturnian/Lunar figure (like Sin). The Hadad system has Ptah/Kothar-wa-Khasis as a craftsman, and El is the Saturnian figure. These just seem like two different systems.