1999 Jeep Cherokee low voltage possible short

1999 Jeep Cherokee mileage: 100,000. My son has a Jeep Cherokee that we have done a good amount of work to. We are down to the last problem and are stumped. The alternator is putting out about 14 volts and the battery is steady at 12 volts. The volt meter in the dash reads at best 12 volts and whenever any light or accessory is used it drops to below 9 volts. We have used a volt meter to check everything and we keep coming back to a short. If you connect a voltmeter to between the negative post on the battery and ground the other lead to the chassis it read 0 volts. Turn any lights or accessory like the rear defog and the volts go as high as 6 volts. This would indicate a short. The problem is we can not find it. It has to be in a main line because any item that uses power adds to the short volts. Any help would be great.