This is on my 91 t2 with haltech and janky wiring.
Was cranking the car over for less than a second and heard a pop and assumed something shorted, then i had no electrics at all from headlights to fuel pump. ive checked all the fuses, main fuse (the only thing i could think that would controll ALL electrics) is getting zero voltage, battery has strong 12v. ive searched with no help, ill keep looking, but thought some of you guys could toss some ideas out. Thanks!

 

loose bolt at the main fuse, and/or loose battery terminals

 

When that happened to me, I had left the primary ground to the starter off.
The bellhousing to firewall ground picked up the load & popped.
No ground = no volts anywhere

 

Check the voltage at the battery terminal connectors. Then, check the voltage at the fuse box. Still 12V? My guess is battery terminals.