I just picked up my car from thebody shop and noticed that the replaced rear window was an aftermarket window and not the original equipment manufacturer (OEM) part. I asked thebody shop about it and they said thatthe insurance company would not approve the OEM part. Can I make theinsurance companypay for the OEM part and get that placed on my car?

Maybe.  First, the insurance company is required to identify each after market part used in the repair estimate and attach a disclosure statement that aftermarket parts were authorized in the estimate.  You can request that the repair shop use original manufactured parts, but you will have to pay the difference in the cost of the original manufactured and after market parts.