Yes, that is what it will be.
Does your pickup still run? Is the battery charged?
Did all the lights work on the service bed before you started this project?
Was the service bed wiring spliced into the original wiring harness? -- in the frame rail behind the cab?
Is the original wiring harness still in the frame rail of the bed (trailer?) you bought.
If the service body was spliced into the original harness (and the answer to the other questions is yes), it will be a little "easier".
To start testing -- turn the ignition key to the acc position (to the left if the ignition switch is in the dash; towards you if it is in the steering column). Do not turn the key to the run position -- position the key is in when the engine is running.
Turn on just the parking lights.
Then, find which wire(s) are hot (have current) - reason for the volt ohm meter - at the splice (hopefully near the back of the cab) or end of current wiring on your truck.
Connect a jumper wire to the hot wire and then connect / touch it to each wire from the bed. See what / if anything comes on. Make note of "what worked" by marking each wire end using masking tape.
Put newbulbs in the taillights so you know they are good (and clean up the socket if it is corroded). Looks like the license plate lights are missing in your step bumper. You need to find replacements -- probably a trailer supply store would be the best bet, but they might be in the trailer wiring / hitch section of a good auto parts store.
You also should get a supply of fuses for the taillights (fuse box is behind the glove box door.) as you will probably blow a few if you ground your "jumper" wire while testing.
Once you get the taillights to work; turn on one of the turn signals and repeat the process. Then, put a brick on tjhe brake pedal to turn on the brake lights and repeat...
Until all the lights work correctly. Then, make permanent connections running the wires inside the frame rail.
If the taillights on the bed are not grounded; then, the bed lights will not work.
Reread michael's / ron's posts..
It May be easier to find a good shop that does electrical / rewiring work; put the bed on and take the truck to them. I do not know how long it will take -- 2, 4, 8 hours labor (do not knwo the "size" of your wiring mess) plus "parts".
