Its a 78 Caprice Classic with no mods. When you switch the A/C on the compressor cluth doesn't kick on, But if you run a hot jumper to it the clutch engages. The Compressor isnt leaking nor iss everything else. Is there a relay or switch that causes the clutch to kick in when switched on inside the car? What else could be wrong?

You're low on refridgerant. Either get it refilled, or convert it to R134-a and fill it yourself.