Help!!!!! Someone, anyone, everyone. My 1989 Chevy P30 Regal has failed me. No exterior lights, headlight, tail-light, hazard light, even my fog lights won't work, also the horn is silent. Driving me nuts. I have checked the current from the Isolator to the fuse box and through the fire wall, but lose it there. No current to the light switch or the horn relay. I have checked the grounds and fuses, have replaced all relays, still nothing. Any help would be greatly appreciated.
Ken Robbins

Those devices should be connected to an always-hot bus.

While you may have checked and replaced all the fuses, there is one frequently overlooked: The fusible link. This is a likely a nondescript metal rectangle with rounded edges, about 1½" long, ¾" wide, with two terminal studs. It will be in the hot side of the battery, and before any other devices.

It would shut down all the things you mentioned if it blew.

First take the isolator out of the circuit. Have the fat + wires from the battery, the starter, and the alternator all connected. That removes the isolator and its grounding as one of the unknowns.

Get an ohmmeter and a length of wire and see if the +BAT contact in the fuse box has contact with the junction of those fat wires I just noted.

If not, then perhaps there was a frayed wire that got burned out or a fusible link that fused.

How did this happen? And, does the starter engage when you turn the key?

I'll have more ideas after I sleep.
Matt