I decided to post your PM response in order to let others in on the topic if you don't mind...
You'd need to take off the lenses and detach all of the middle decks so you can inspect all the wiring. Focus on the rotator motors at this point and not the bulbs for now. Chances are you may have some bulbs that are burned out. I can already tell that you have a ground issue when you mentioned dim bulbs. When there are dim bulbs, it's finding ground through other components that also includes bulbs.
The decks are grounded by their own fasteners to the frame so testing it with the decks apart just won't make it work. You can use a temporary jumper or simply touch the deck to the frame for a quick test. You won't harm it with this method. Just keep clear of the rotators.
Ensure that the lightbar's black ground cable is connected securely.
Check each wire and verify that it's connected securely to the lighthead(s). Are they factory wiring? If not, you'd need to inspect closer.
When you get everything to work, double check the suspect bulbs by inspecting the base. A batch of bad bulbs caused the tip to melt, shorting it to the ground. If that's good, swap places with a good bulb, this will tell you very quickly of it's a bad bulb. If the problem exist at the same place, you'd need to double check the base. It should move freely in the housing to provide a solid contact.
Check the rotators, it should have absolutely no contact with any part of the deck, wiring, etc. It's not uncommon for folks to blame rotators when there isn't sufficient power. They use a lot of juice, hook it up directly to a good battery for testing. Rotators need a minimum of 14 gauge, each lamp 16 gauge. Pairs of rotators on one wire should be 12 gauge.
Original factory wiring for the intersectors, especially smart intersectors are inadequate and require bumping up a wire size.
When replacing wiring, do follow the existing harness especially to the decks as it should be only one bundle of wiring to facilitate ease of servicing (service slack). When reseating the decks, take care not to pinch any wires. Half of the problem can be traced to pinched wiring.
Replace any "browned" connectors, it's a sign of overheating and will fail. Install new quick-disconnect connector(s) at one or both ends if you can't find a mating piece. It's a good idea to use dielectric grease on the connectors, a little dab before pushing the connectors together will go a long way.
When all's done, tell your buddy he owes you a six pack for the trouble!