If you can't get any lights from it while internal or external it must be defective.
I don't know where you purchased the unit but you may want to just ask to exchange it for a new one.
Aside from doing that, I went to xlr8yourmac and looked into the reports for Pioneer and AGP G4s. There were many people who had successfully installed the 112D with one who got a bad unit and needed replacement.
Looking through all the other reports the following models were reported as being successful by at least two users each:
110D
111D
There was one listing of a 109D being used.
One problem often mentioned with some of these was they had to remove the shield on the back of the unit to get it to fit.
I did find some positive reports of the 108 being used in Macs but they were from 3 years ago and those units don't have all the features of the newer drives. If you found one it would probably be used or at best refurbished.
OWC lists the 112D as being compatible with G3, G4, G5, etc. so I can't see why yours isn't working other than the fact that it may be defective.
