HELP! 2001 Volvo dayCab shorting out somewhere?

New to me Volvo truck with a Cummins ISM 370 and Eaton fuller 8 speed.

All the sudden gauges started going crazy
Ac clutch going in an out
Fan clutch in an out
Just crazyness

I’ve checked out everything thing simple, looking for shorts etc. I’m new to working on big rigs so I’m learning (but work on my 06 Dodge common rail pretty often).

Each battery shows nearly 13 volts and there only a few months old. The truck cranks fast and starts right up.

But the volt gauge on the dash only is showing around 10, going up or down. Lights get dimmer, then brighter, heater fan barely will blow (not enough voltage).
Like something is drawing way to much power, or something in hindering it.
Wondering where to go from here
Thanks!

 

Ok, so I found the post at the firewall, 13 at it.
Then checked under the dash at the fuse panel, 13 at the first post, then 13 on the other 2 when I turn on the key.

 

Both

 

Wanted to post back the found problem. The ground terminal on the firewall (in the first picture) wasn’t getting good connection. Cleaned it and volt gauge now showed 13 volts with the heater fan on and everything is back to normal. Thanks for the interest guys