I compiled this list long ago by going thru the ’86+ fuse panel wiring diagrams to help people with questions about powering extra instruments and other accessories. It’s been copied many times by others, usually without attribution, which is pretty rude, because it took quite a bit of time to write up so credit where it’s due. After the first time I posted it, I later copied it to a thread helping someone wire up a stereo head, so I’ve included that advice as well, and added a bit more detail throughout. So here is all of it for your edification:

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When you want a power supply for an accessory, there are two groups of unused male quick-disconnect pins on the back of the main 86-and-later fuse/relay panel for most anything you’d need.

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On the left end (as always, driver’s directions) of the panel there are 7 pins in a group labelled “P”, in two vertical rows of three each plus one more at top. These are all #30 pins, which means battery direct, unswitched, unfused, and common with each other. These are good for fairly heavy consumers.

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The lowest two are wide 9.5mm/3/8″ pins, one is occupied by the main power feeder from the battery positive pole, the other by the main feeder to the ignition switch. The other five are 6mm/1/4″ pins.

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(Note: the main battery feeder connection there carries all the current the panel distributes, so it’s quite common to find that female connector showing signs of overheating and loosening, causing a myriad of intermittent electrical problems and the potential for an electrical fire, so replacing it with a fresh 9.5mm female connector should be a routine step).

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At the other end of the panel are ten 6mm/1/4″ pins in two vertical rows in a group labelled “G”. These are hot under various conditions and can be used to power accessories. You can patch into the panel at whichever pins do the job for you by just plugging on a female quick-disconnect terminal.

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I’ll list the G pins according to the diagram. Where power is via a panel fuse, I’ve noted that with the letter “S” (for “sicher”, German for safety or fuse). Always verify with a testlight or VOM before hooking up:

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G1 & G3- X-bus load-controlled power via S12 (X-bus is hot when ignition on, but goes cold during cranking)
G2 & G5- #15 ignition-switched power via S18 (this is the ideal power source for extra instruments)
G4- D+ (alternator exciter circuit) via alternator warning LED
G6-dead end
G7 -headlights power before hi/lo beam selector, so hot during both
G8 -dimmer-controlled panel lighting power via S50 fuse in clip-on fuse socket above panel (ideal to light extra gauges)
G9 -parking lights power via S20 feeding license plate lights (for your low-rider under-chassis lighting, ese!)
G10- hot when windshield washer pump runs (used for headlamp washers)

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It is always advisable to use a fuse inline from any power source to protect your device and wiring. If you are taking power from a G-pin that is already fuse-protected, be mindful of the current you are adding and whether you may need to increase the fuse rating.
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There are many ways to set up the radio, according to peoples’ tastes and habits. The stock wiring made available to power an in-dash radio is on the always-hot #30 bus, via fuse #3. This circuit routes power to the radio, interior lights, clock, and cigarette lighter.

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Many folks instead take radio power from some #15-fed connection, or the #15 bus in the main panel (G2 or G5 listed above). The #15 circuit is the main ignition switched bus that powers up all engine controls and some accessory circuits. This way, the radio goes off when the ignition is switched off.

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In addition, there is a way you can have your radio come on only when the key is inserted, but the ignition doesn’t have to be switched on. Pull the key out, radio is off. It is the “SU” signal connector that goes to the seatbelt warning relay (my guess would be for “schlussel”, the German word for key). The seatbelt warning relay is mounted in a clip-on relay socket atop the main fuse panel (position 14 in the panel diagrams). The SU wire is gray/black. When the key is inserted it closes a tiny contact in the ignition switch that makes SU hot. I use this method myself.

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If you have removed the warning relay, you can use a 6mm/1/4″ male spade connector and just plug into the SU pin on the face of the relay socket. If you wish to keep the seatbelt warning, you will need to tap into the gray/black SU wire on the back of the relay socket, or where it emerges from the ignition switch.

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If your radio has only one power wire, and you use the radio’s internal amplifier rather than an external power amp, you should use the SU signal to close an independent relay to power your radio, as the SU signal circuit has very low current capacity. If you use an extra amp, your radio will use very little current directly, and its trigger wire will switch on your amp, so you can probably get away with using the SU wire directly. Your separate amp will need its own power feed, amps use high current so this should be a fused feeder from #30. My setup works this way.

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Also, if your radio has two power input wires, then one is intended to be connected to #30, so the radio’s station presets and settings are preserved. Radio manufacturers differ on how they do this. Some use the #30 connection as the main power feed to the radio, and the other is only a trigger. Others use the #30 as only a small-current feed to keep the memory hot, while the switched wire is the main feed. One way to tell is to look at the fuse ratings for each input wire, and the relative size of the wires. The bigger fuse or wire will be the main power feed. If that is also the wire that they recommend be switched, and you do not also use an extra amp, you would be wise to use SU to trigger an extra radio power relay, rather than overloading the SU circuit.

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If you install an SU-triggered relay, you can of course use it to power other accessories that you would like controlled by the key-in function.