Mazda MPV van won't start

I have a 1989 Mazda MPV Van that won't start. I checked the battery and it's putting out 746CCRs out of a 700CCR battery and I had a load test performed and that passed so I thought it was the starter solinoid so I replaced the starter and it still won't start. When I turn the key to the run position the lights on the dash come on and if the door is open the dome light is on but as soon as I turn the key to the start position, even for an instant, the starter clicks once and all the lights go out even when returned to the run position. It's as if the battery shorts out instantly but if you let it sit for a while it does the exact same thing, lights come on in the run position then one click in the start position then everything is dead. This makes me think that it's a relay but I thought the only relay was the solenoid on the starter. And why would the whole cars electrical system go dead? Is there another relay that's activated in the ignition circuit? If so where is it located? I would really appreciate your help as this is my only means of transportation. Thank You Very Much. Davedurr@live. Com