If there is only one used available then I recommend you do not risk that glass on a learning experience of how to remove w/o breaking.You could try offering your installer $ to go and remove it for you,or ask at the yard who they use to pull glass and try that person.Glass is very tricky with both breakage and leakage,it sounds to me like if you have a local experienced installer do both parts of the job for you,you will save your self time and around $650.00.
If you feel like you really want to attempt this extraction I could offer several tips.
My first tip would be to read the 700/900FAQ section on w/s removal,the back glass is very similar(the part about how to insert the wire,do not use a screwdriver,multiple sweeps with a razor knife is safer for the glass and will create a nice entry point for the wire).
My second tip would be to practice on other cars.
good luck