I've taken off the cover and the wires are all OK but the black one where does it go to?
My heater won't come on (well some times the AC come on with out me turning it on.)

The black may be a common. With no more details, I suggest you go to the sticky at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html