What these guys stated is where to start. Bad battery connection most likely. If it's not that, very possibly could be a bad bulkhead connection. The two bulkhead connectors (at the firewall under the master cylinder) can build up corrosion. Simply unplugging them, cleaning the terminals with electrical cleaner or WD-40, and plugging them back in will often fix the issue. When the connectors get too much resistance, nothing will power up. Good luck.
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Jeff 
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