Sounds like you could use a manual for that truck. Anyway, without making this confusing - you have 4 batteries in that truck, but they're all hooked together. Radio runs on 24 volts. You need to find which of the '-' negative terminals has the battery cable that goes straight to your frame. That's the main negative, and your radio power cable black and green wires go to this terminal.
The main positive terminal will be a little tricker. It will be the "+" terminal on one of the batteries, and the thick cable coming off it it should go straight to the starter. Hook the red and white wires from your radio power cable here. That should do the trick.
Alternatively, if you have a slave (jump-starting) receptacle fitted, the two thick cables running down from this will go to the main negative and main positive terminals of the batteries that you need to hook your radio power cable up to. Pretty sure all M816s have this fitted.
