95 k1500 power issue
 

I've got a 95 Chevy k1500. With the engine off the battery voltage rests around 12.5 volts but all the power inside the truck sits at around 9-10 volts. I've grounded my volt meter to the battery and checked the fuse box and all of them stay around 9-10 volts. But with the engine running the battery had about 14.3 but the inside only has about 12 volts. I'm not sure if there's a voltage regulator to the inside or what it might be. Any help is appreciated