My 16 year old's first car!

[MadMan]

The trans was rebuilt? By who? Might be past warranty by now, but still, ask them wtf happened.

No forward... did it ever work after the rebuild? Because if it did and you drove it, I'd be tempted to say it might have chewed up a band or something. I'd have to look up the chart to see what's on when it's in drive. I assume you've tried moving it in 1 and 2. Smell the oil, it shouldn't smell burned if the unit was just rebuilt. If the fluid is still clean and fresh, it might be something simple like a stuck valve. Especially makes sense on a project car that probably sits around and never gets driven.

If the oil still seems fresh, and the car can run good enough, start it and let it run for an hour to get the trans nice and warm, see then if it'll engage.

Let me know a little more and I'll see what I can come up with. It's not easy when I'm not there to see it. :/