Ok, got you.

Here's how this works - battery cable to starter w/smaller wire back to main breaker then regulator output and main feed to main switch on other post of breaker. Main switch feeds #30 on that relay in all switch positions except lock or off. #87 is trigger wire for the starter, #87a goes back to the ACC breaker under the dash. Meaning, volts run from main breaker thru the main switch to and thru that relay back to the acc breaker.

If you're only getting 11ish volts those 2 places (under the dash and on 30 and 87a) with full bat volts on the main breaker, it has to be the main switch, wire between it and the breaker or something close to it. You could put a temporary jumper from the battery to #30 of the relay and with the switch on, if everything worked with full volts everywhere, that would verify the guess I've just listed.

Way past my bedtime so lets hope it's not the main switch itself. I'll be having nightmares about changing one of those switches... and even finding a replacement is 10 times harder than that.

 

I believe post #30 comes from the ignition not the battery, but it is positive.
If you're reading 11 volts there it's a bad contact in the ignition.
Pull the positive off the battery and check the roller contacts under the ignition.

 

There is also a cheesey little connector in the orange wire going from the ignition switch to the rest of the bike, near the switch.

 

I had the low voltage in the fairing and acc circuit problem, it's a bad design running all the load back and forth 3 times, also through the starter relay and ignition switch, there's also no water tight connectors and the moisture compromises the terminals and wiring over time.

What I did was add a circuit to bypass all the old wiring. Ran a new wire, fused near the battery, to a relay near the circuit breakers in the fairing, used the tan wire to trigger the relay, the relay then powered the circuit breaker.
Later added another circuit like that that powered both the radio and ignition circuits.
Also added a relay next to the starter relay, powered off the main circuit breaker, to shortcut the power to the starter solenoid, at that point the old starter/acc relay only trigger the new circuits.

Ran it like that for a long time until I built a more modern wire harness.

 

FLH/T used the 5 pin starter relay so the acc circuit (#87a) is disconnected when the starter (#87) is energized, really simple actually.
That cheesy orange connector comes off the main switch, connects to the harness wire that feeds the start relay at #30.
You can do interesting things with extra relays for sure.

With these bikes, electrical gremlins can be be hard to find, but it's always something simple when you finally do.

 

I was up too late, not thinking logically.

"Voltage under instrument cluster is 11.5 volts"

Where exactly were you checking voltage under the dash? The power wire from the main breaker terminates at the "constant" breaker on the far right, looking under the dash. There are 2 red wires connected on the bat side - one is the main feed from battery, the other is the feed to the main switch. If the connections/terminals are good on the main breaker but low voltage everywhere else, probe/check/clean that connection on the far right breaker and see what you get.

 

With the key on and not running I get the following

12.14 at battery
12..04 at red wire main breaker
11.81 at the far right breaker under the instruments. Red wire coming from main breaker
11.4 at far left breaker under the instruments. Orange wire that I assume feeds the accessory circuit.

If I pull the brake lever the last one (orange wire) jumps down to 10.1

 

Just ran a ground from the battery cable location where it attaches to the starter to another part on the frame, and ran a wire from the ground block under the instruments to that ground location. Even ran a wire straight from the positive of the battery to the accessory circuit and still only getting a little over 12v at the gauge, and still getting the dimming, and huge drop in voltage at the gauge when I use the blinkers or brakes. Weird...

 

I don't think it's a ground problem. In my opinion, it is the numerous connections between the battery and everything else. I think I counted over a dozen between the battery and the starter solenoid, for example. Each connection, including inside the breakers, provides some additional resistance over what a straight wire would have. With the brake lights on, check the voltage at each of those points. A resistance at that point will increase the voltage drop under load versus what you see with no or light load.

You could also experiment with some clip leads and jumper across a breaker and see if the voltage at the end orange wire comes up.

I think my final solution would be to do like Schex suggests and put a relay in.

 

For reference, breakers under the dash left to right are: Acc, Lights, Ign, Constant. From the factory the Light and Ign breakers are jumped on the load side so they burn all the time ign is on. Many of us disconnect that so lights are off for starting or running dark. If you do that, you'll have to run the main switch in the lights position to be legal on the road. You mentioned "turning on you lights" so it may already be un-hooked. ?

A few things to keep in mind - the fairing volt gauge is at THE very tail end of the acc circuit and that particular gauge the factory used isn't the highest quality, they give out over time. I'm on # 5 with my '87. Yes orange is acc feed throughout. It runs the brake lights, gauges, instrument panel lights, turn signals, radio main. Also, there is an anti-dive valve for the front air forks at the ground block we've discussed. You'll hear a loud click whenever the brake light is activated and it pulls a lot so all that adds up to a lot of amps on the acc circuit. It's not unusual to see "some" bit of dimming when the brakes or T/S are applied.

From the readings you posted, you have a bit of loss from the main breaker to the connection at the constant breaker where the switch gets power and excessive loss from main switch feed (thru its journey) to the acc breaker. Here's where I'd start - remove and clean both contacts and all wire terminals on the main breaker. Do the same with the one (or both) connection at the constant breaker.

Load test the starter relay. Meter on lowest DC volt setting, back probe with meter leads at #30 and #87a with switch on. It will read some fraction of a volt and that will tell whether the feed thru on the starter relay is choking things.

Hess just wrote a good post while I was (slowly) typing all this but I'll post it anyway. Bottom line - the acc circuit has more routing, connections and load that everything else on the bike combined. Add the age of components, connectors and such, sometimes an extra relay under the seat as Schex suggested is the better option. I've done quite a few and it's simple, easy and clean and keeps everything working just like factory. On a '89 up, usually only have to pull the dash and seat, not the tank.