I have a Sub Zero 642 Freezer runs continuously and won't turn off when unit is turned off. When I turned the whole unit off at the control panel the freezer compressor continued to run. The freezer is currently set to 0, but the temperature is at -17. I turned off the breaker and defrosted the whole freezer, but when I turned it back on the issue is the same. The unit was already in the house when we bought it in 2008 so I'm unsure of the age. We have also recently had the fridge condenser replaces last year due to a leak.

What could the cause of the issue be? Is there a relay somewhere that is stuck on? Where could I find a wiring diagram?

The thermostat is stuck in a closed position. In other words, the contacts are welded shut. You need a new thermostat fast or unplug it right away.