Yep. 85 and 86 have to complete a circuit from power to ground. So +ve can go to either. And the other one has to be earthed.
But this is where the trigger comes into it. Eg:
+ve trigger goes to 86 and 85 has to be grounded to the chassis.
Or
-ve trigger goes to 86 and 85 has to be from a 12 volt switched( meaning only when key is on)
When 85 and 86 have a completed circuit this energises the coil and pushes the circuit closed between 87 and 30 (the coil is like a reverse magnet, which pushes the bridge closed as per 89cal's diagram above)