1996 Other Jeep Models Auto shutdown relay keeps blowing

My 96 Grand Cherokee V8 has been blowing the 20A fuse that serves as the automatic shutdown to protect the ECU. The dealership tech says it must be a short in the injector harness somewhere. With a new fuse installed and with it running, a code reader indicates no problems in the electrical system except the second o2 sensor is giving erratic readings (jumping from 0.00 to 0.80). The only other thing I see is that the truck doors will lock but the alarm indicator light has stopped working and doesn't flash when locked. This truck is in very good condition and was garaged every day for the first 8 years after purchase. At this point, other than installing an $1100.00 new harness, all I can do is carry a box of 20A fuses and replace them as they blow. Does this problem sound familiar to anyone?