It looks intimidating but...
It looks intimidating but this schematic reveals the secrets of retained accessory power. Copy it to a tee (especially the diodes' direction) and it will work. It can be used to power the whole accessory circuit but I advise limiting it to as few devices as necessary.
The basic turn signal-only version requires four SPDT relays, a two-wire flasher (if your fuse panel doesn't have an integrated flasher), and one latching SPDT on/off/on switch. A toggle or rocker will work; however, this setup is a great way to resurrect half-working turn signal switches. As long as at least one contact on both sides is good they'll work perfectly with these relays.
Naturally, you'll want to fuse all inputs. It bears mentioning that the relays will drive the brighter filaments (brake filaments) in dual-filament lamps.
If your fuse panel has a hazard output, feed it to terminal 87 on all four relays. If not, feed constant power to a flasher's input and connect its output to terminal 87 on all four flashers. Likewise, feed switched power (ignition) to the common (center) pole of the toggle or turn signal switch. One output leg of that switch feeds one pair of relays; the other leg feeds the other pair.
The clever high-beam relay...
The clever high-beam relay comes in several flavors for Porsche, Audi, Volvo, BMW, and Mercedes-Benz but the most popular and affordable is VW PN 111-941-583. It looks like a regular relay but its terminal pattern doesn't fit universal sockets. Wire it with individual push-on connectors.
A very slight modification will make that turn signal system operate as a four-way flasher. Simply feed constant power to a Double Pole Single Throw (DPST) latching switch and connect each of its output legs to an output terminal on the turn signal switch. The two sides remain isolated from each other when switched off; when on, it feeds power to both.
To make single-filament, front turn-signal bulbs operate as combination running lights/turn signals, connect the running light or taillight feed from the headlight switch to the front relays' 87a terminals. For a simple dash indicator light, connect one leg of a lamp to constant power and piggyback the other leg to the output side of the flasher module (the side that blinks).
RAP Anyone Can Like
I'm proud that the newest car we own is old enough to vote but a new car once made me jealous. Its radio and power windows remained operational after I removed the key and stayed that way 'til I opened the door.
Retained Accessory Power (RAP) made me actually consider financing a new car. Then I discovered www.the12volt.com, an automotive electrical site chock full of neat tricks and solutions. It has a relay network that recreates the RAP feature with three relays, two $0.50 1N5402 diodes, and door switches that are probably already in your car.
Connect terminal 56 to the...
Connect terminal 56 to the headlight switch. Connect terminal S to ground through a momentary switch. When wired this way you can flash the high beams even when the headlights are off.
The RAP relays require power from the accessory circuit and common door pin switches. The ones that trigger your dome or courtesy lights will work fine.
The system can be configured entirely with SPDT relays but its power will be limited to about 10 amps (which is still respectable). To increase its power, swap the first relay with a high-current SPST relay and you can probably power the entire accessory circuit if you wish.
The system energizes when the ignition switch reaches the accessory position. The accessories will stay on as long as the ignition is on regardless of door position. But as soon as the ignition turns off the accessories will stay on only until a door opens. Turning the key back to the accessory position (or starting the car) will re-energize the circuit and repeat the cycle.
Most relay coils don't care...
Most relay coils don't care which end gets power or ground but ones with diodes do so make a habit of using 86 as the coil power in and 85 as ground. Either and/or both side(s) may be switched. Terminal 30 is common, meaning that it can contact either 87 or 87a (if a SPDT relay). When the relay is in its neutral state, normally closed (NC) indicates a completed circuit with 30 (as in 87a); normally open (NO) indicates an open circuit (as in 87).
One-Touch Headlight Dimmer
Some of the German cars I grew up with had dimmer switches that operated much like GM switches: pull back once to activate the high beams; pull back again to go back to low beams. The Krauts' system differed in one novel way, though: rather than use a clunky mechanical switch that won't adapt to anything else, they used a relay that can be activated by any momentary switch.
As soon as I figured out how to use those relays I went about replacing floor dimmers with them and retrofitting turn signal stalks with tiny hidden grounding switches. Technically this relay doesn't fall within the definition of universal but it's just too good to ignore.
A quick reminder about terminal S: It works by grounding momentarily. Grounding it for more than a few seconds will overheat and possibly fry it. As a belts-and-suspenders type of insurance, consider splitting each output (56a and 56b) into two wires and route each leg through its own fuse. That will eliminate the likelihood of losing both headlights or beams if a problem arises in one circuit. Painless Performance carries a few stand-alone four-fuse panels, including the very affordable four-circuit ATO fuse center (PN 30002).
Mini SPST relays can handle...
Mini SPST relays can handle as much as 70 amps, perfect for simple, high-draw applications like fans. DPDT versions are usually rated by each throw. Relays like this with tabs are self-mounting so can use plain sockets (shown).
Micro relays are about half...
Micro relays are about half the size of mini relays. The flip side is that they handle less current-some as little as 10 amps. That's still fairly healthy, though; a 10-amp current at 12 V works out to 120 watts.