1978 GMC Astro 95 Cabover

Hi everyone, first time ever trying one of these threads. Have a couple trucks and just bought a 78 Astro 95 with a NTC 290 Cummins. It has 3 stage Jakes, although the switch doesn't activate and I can get them to switch on if I run a wire to reground it somewhere else, so I realize this is a really stupid question but I've looked everywhere and cant find an owners manual. But for the life of me I cannot find the fuse box. The grounded wire runs to the amp gauge and that does not work either. So someone who has worked on these has to know where I can find this. Thanks

 

Thank you, so I did remove that cover and when i do, it reveals the back of the cluster and switches and the wires lead to somewhere further in the doghouse, nothing looks easily accessible to remove. I am going to look at it once more later and will try to upload a picture. I just feel like once I removed it I'm at a dead end.