How can I turn my rear window into a one-way mirror?

This easiest way to do this is by using limo tint or one way mirror window tint.

The thing to be careful of here is to be ware of local laws which may be at issue with your designs. At least in the US, most states require that you do not put this type of tint forward of the B-pillar (post at the rear of the front door) so as to not impede the driver's vision.

In many ways, a better solution would be to install an auto-dimming mirror:

• most auto manufacturers nowadays offer this feature as an option
• the dimming gel relies on light-sensor input to decide whether to lighten or darken
• installing window tint reduces night-driving visibility
• not all legislations approve of the installation of window tint.

_{(source: gentex.com)}

Auto-dimming mirrors do require an electrical power supply, but should be quite easy to retrofit into a vehicle which has interior cabin/reading lights, since the wiring for the lights can be spliced into.