It's most likely a 1986 rear hatch and you can probably prove this because there is most likely a three panel electrical connector on the hatch that you don't have on your car. The rear light was mandated by law before yours was built.
The '86 hatches are less desirable and therefore was probably just sitting around because no one wanted it.

Try to get your deck lid back before saying anything as that will match your paint and can be transferred to the new glass. Local junk yards often sell these cheap (Ie. $30 Amer.) So look around if they're trying to screw around with you.
Don't flip out. Prove your case and work with them as much as possible to correct things.
Also 1989 rear light: