Need some electrical diagnostics help

I could use some help as I’m stumped. I’m slowly going through the truck ( 03 Mack cl713 tri axle dump truck) to get it on the road hopefully within the next week and start hauling with it. So on my box, I’ve got 4 amber marker lights that are out, but 2 red one towards the rear that are working. I’m assuming they are all on the same marker light switch, but am not sure how to trace them to find the issue?
Second is truck didn’t have a working backup alarm, so I get a new one connect it up and it’s not working, this may sound like a dumb question hut does the truck need to be on or key on power enough?
The trucks wiring in the back is a mess tons of capped wires, I’m not really sure even how or where to start. I’m looking for a good way to attack this.

 

I was thinking a test light. Only issue is the battery box is on the front of the truck. The box is 17.5ft long so I would need a very long tester. Not sure how else I could do it, do they make cordless testers?

 

yes key on and in revere, there’s 4 wires ( 2 sets) that are coming out of the junction box at the rear that I’m assuming is for the body, and couldn’t get the alarm working.

The 2 marker lights on the rear side of the box come on when I turn on the headlights, I am guessing the others would come on as well, unless they are on the marker light switch, but I believe that’s just for the marker lights on the cab. The 2 round marker lights next to the strobe and the one near the door are the ones I cannot get working ( replaced with new ones) but the red one in the back circled comes on. If that makes sense

 

I don’t know much about electrical, but I trying to learn, but at the same time I could spend 40 hours looking at it and chasing wires and never get anywhere where as someone who knows what they are doing mah be able to do it a whole lot faster. anyways.. this is what I’ve figured out so far,
When I turn the lights on in the truck the headlights, brake lights come on. As well as the Lights on the box closest to the rear ( the check mark in the photo) the rest that have the x do not come on. I’m assuming that a wire or something runs from the light switch all the way back to the junction box and then the brake lights, and then signals as well as the marker lights for the box come from that. The rear most lights are working so I’d say it’s getting power but something between the lights and the front Lights have an issue, I’m off to get a test light and probe around it tomorrow and see what I can find