ACC cable in 1999 ford expedition!?

The acc (cable?) wire from your stereo connects to the acc wire of the vehicle at your dashboards area that houses the stereo.  Can't really explain it any better then that.

If you are using an aftermarket harness adaptor it is color for color, in this case red is acc.

You can purchase the correct adaptor for your vehicle anyplace that deals in car audio, it is a popular vehicle. That will make things go alot easier for you.

If the aftermarket harness is not showing acc, check the fuse at the factory fuse panel(s)

If for some reason the acc wire at the dash is dead, and all the fuses check out, you have a bad (broken) wire. The best place to re-run it is from the ignition harness. Use a meter to determine acc, fuse it close to the power source, and safely run the wire (16ga. should suffice) to the dash.

If the factory stereo harness plug(s) have been cut, hacked, chopped, ghettoed, masicared or otherwise butchered off, and you do not know the proper way to determine which wire is acc ,I can best advise you to have it professionally installed or if you know someone you can trust to help I feel the install will be more of a success.

Just the fact that you asked "where" it is leaves me leary to simply post the color codes.

However, If I am misunderstanding what you actually mean, please repost and accept my apology.

Hope I helped somewhat..