"1) How do you disconnect the supplemental power supply to the airbag?"
It is a capacitor. Wait 15 minutes and the power should dissipate.
"2)Do you have to remove the steering wheel to access the clock spring?"
Yes. If you need to remove it. If it is just a connector
which came loose, maybe you can just reattach it and hope that will solve your airbag light/no horn/no cruise issue.
"Can I get this part from like a NAPA or Autozone?"
I think you need to get it from Ford.
Here are the Ford CD instructions to remove the clock spring/air bag sliding contact. Good luck.
Sliding Contact Air Bag
Removal and Installation
1. Make sure that vehicle's front wheels are in the straight-ahead position.
2. Remove steering wheel and driver side air bag module as outlined.
3. Remove steering column shrouds as outlined.
4. Disconnect air bag sliding contact wiring harness.
5. Apply tape across air bag sliding contact assembly stator and rotor to prevent accidental rotation.
6. Release three air bag sliding contact locking tabs and lift air bag sliding contact from steering column.
7. To install, reverse Removal procedure.
