No Power in the cab

I am in the process of trying to Fix my 1990 740 wagon. This car died on me in town. I have towed it to my home and am trying to find out why it has no power inside the car. I have charged the bat , cleaned the cables , checked for power at the alt and starter. The starter will crank if I jump the solenoid and the alt has power. Their is no power at all on any of the connections inside the car. No dome light , No dash lights , no change with the key on or off. The

4 way flashers don't even work. The fuse panel is completely without power. I can find no trace of power inside. It is like the cab has been unplugged. The day it died I started the car and it ran fine. I turned the heater on then the lights . At this point all went dead. It was like the lights pulled to much power and something clicked off and took all power with it. After a short time the dash came back to life and the car once again started and ran fine. Upon turning on the lights still fine then when I turned the heater on high again it all died. again after a short time it all came back so this time I left the heater on low and the lights on parking lamps and all was fine. I went to town made several stops and turned the car off as many times . It always started again with no indication of trouble. Starter sounded strong , car ran fine . I stopped at a stop sign and this is where the car went dark , all power inside went out and stayed out. I have been searching for the source of this power outage for 3 days now with no luck. I can not understand why a loss of power that disables a car would also take the 4 way flashers out, as a disabled car really needs the flashers at that time. I am willing to look at all suggestions at this point . Please help. Thanks.