freightliner electrial issue

so I got a 2007 Columbia with a c-15 cat all my interior lights are dim and I am only getting 8.2 volts on the iso wire in the fuse box now this is what I have don so far I cleaned all the ground wires yes the one in the floor on the drivers side behind the kick panel too cleaned the battery to checked the hidden 50amp fuse on the pass side floor behind the harness replaced the lvd and all the fuses in the battery box only the iso circuit has low volts so I put a jumper wire with a 10 am fuse in it that lasted a week and it burned the fuse out I don't want to bridge this circuit for fear of burning the truck down I have searched all the places I can think of where the wires may have melted together or could have been pinched now if I turn the head lights on with the doot open and interior lights on they get bright then go dim again and yes I checked the fuse box behind the drivers seat too and I been doing this a long time and do most of my own repairs I have been over every inch of this truck the only thing I can think is a hidden wire issue there are no xtra lights added or any extra things added inside like tv or microwave its all factory one thing I did find is a 25 amp fuse in the fuse box with power on both legs or both sides with the fuse removed still trying to find out what that controls i have seen this problem on other web site but most of them complain of no interior lights in cab or sleeper I have power for interior lights and power outlets cab and sleeper it just not 12 volts it is 8.2volts

 

you are refering to the low voltage disconnect ? if so I replaced it already it is in the dash below the cup holder

 

replaced it already .

 

how many power wires come in to the cab I have 4 power wires on the fuse box 3 of them are showing 12.9 volts and one is showing 8.2 volts some only come on when the key is on the one that's on or hot at all times is the one that's only got 8.2 volts I have been all over this truck and cant seem to find the problem I am thinking I got a wire grounding out some where in the wiring harness its self but not sure where to start looking