Wiring Issue

[Maroon_Madness]

Hi everyone I'm new here, I recently picked up a 85 blazer that has been sleeping for 12 years in a barn. Today I got the 350 moving and when I went to try the starter it cranked for half a second and everything went dead inside the truck. no lights, gauges etc. Checked the battery and it is still good (brand new battery) I tried it after ten minutes and the same thing happened. guage said I had 12-13V before trying the starter. There was a mouse nest in the air cleaner but the wires look fine around the motor. Does anyone have any suggestions as what to check first or some troubleshooting tips? Also this is my first real project truck, I love wiring but never had to deal with a short somewhere.