I have not practiced it myself, but there are numerous posts explaining that for the turn signal breaker the bulbs need to provide load for it to work. LED do not provide this load. I do not know what exactly the symptoms are, but turn signals may not work just with LEDs with no resistors. For the same reason as when you put in 21W bulbes vs. original 18W, the turn signal will be blinking slower. When one bulb goes blown on one of the sides, the blinkers blink very fast on that side. This is because of how these old breakers work.
If you installed LEDs as turn signal bulbs without resistors - I suggest you put back bulbs and see what happens.
Difficult for me to verify now which one is pin 12 from your description...
MSmith, you have access to the coloured wiring diagrams:
https://www.sl113.org/wiki/uploads/Restricted/280sl.jpgComponents descriptions are here:
https://www.sl113.org/wiki/Electrical/WiringDiagramSimplest way to check power for fuel gauge and reserve light:
You have the instrument cluster shown on diagram with all its connections (10a, 10b, 10c....). Fuel gauge is 10d, reserve light 10c, but you should look for the power on the one without a letter (black/blue) as wires from 10c and 10d go to fuel sender in the tank. Brake fluid level is 10i (this one I would skip for now or make sure that power is there only when connectors in the brake fluid cap are closed and both driver and passenger doors are closed). You see the corresponding colours of the wires to all connectors. If you look at the handshake connector (harness side) you can identify which pin is for what based on wire colours. Then you will know if you have power on "letterless" pin for fuel gauge/reserve light power supply pin. This is probably the easiest way I can think of now to determine power supply to fuel gauge and reserve light. If no power there, the other end of this wire is fuse 5.
Simplest way of checking power supply to brake fluid level (that is where I would start, actually):
with ignition on check if you have power on any connector in fluid reservoir cap plug (just pull out the plug with harness wires and check in it), ignition on
with the driver and passenger door closed (I know it sounds weird, but there is a reason for that). If not - next step is fuse 5, the other end of this wire.
Simplest way to check power supply for back up and stop lights:
rear lamp bulb holders. Brake light with ignition on, pedal depressed. Back up light with ignition on, reverse gear in. If power is not there, then you would need to move earlier in the path. There are more points there to check, but I would move to the beginning - fuse 5. You will see switches 23 and 24 on the diagram - for later if need be. If power is there and bulbs are not lit - there may be a grounding issue.
To check the fuse 5 wire connection the easiest way is to look up there under dash and check it visually and reaching there with your hand. The wires either hold to fuse No.5 connector or not.
I would strongly advise to disconnect negative battery terminal while getting to the components and put the terminal on without tightening for actual tests only!