The Stern Shaker motor per their schematic operates on 24 volts AC. It comes with a circuit board that has some beefy diodes and a resistor along with 2 fuses and 2 plugs. The circuit board rectifies the AC voltage to DC current and the resistor makes it closer to a usable 12 volts as the motor is rated as such. The board has 2 plugs. 1 plug(2 conductors) to the motor and another plug also 2 conductors for power and trigger (ground). I initially tried to run the motor on a separate small 12 volt power supply since there was no 24 volt AC available in the game but the amperage was not enough.

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Further research found 20 volts AC directly from the WPC transformer to the sound board so I figured a 4 volt difference couldn't be that bad and I tapped this supply line near the J501 header plug with the red power wire from the shaker board.
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Looking again at the stern schematic the other wire (brown), is instructed to plug into some specific under playfield harness but to complete the circuit it appeared to simply need a ground to activate. I tested it on a ground braid and that thing shook like a beast!! However I chose not to connect this wire to any direct sources in my game but instead chose to ground it through a relay to prevent any type of electrical surges or short circuits.
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The next deli-ma was that I knew I wanted the shaker to activate with the time expander. My relay will trigger from a + or - source but not both..it is selectable. So I figured I would tap into the time expander motor power wires and use this to trigger the relay to trigger the shaker right??? Wrong...the time expander motor reverses depending on game play so either wire will potentially have + and - currents at any time which means the shaker would only work half the time the expander is moving. I suppose I could have used diodes to block the reversing current but I did not want to risk damaging the bi-directional motor board. Those things are sensitive and I already damaged it early into my restore while rebuilding my time expander.

Better solution...Looking at the Williams schematic for the Bi-directional motor driver board I noticed one circuit determines the direction of the motor and another circuit simply turns on that circuit or opens the gate for it to function. Tapping into the circuit that turns the whole system on was perfect for my application. This wire plugged into J122 header on the main power driver board. I tapped it there and found it needed a diode to correctly operate as it carried power but switched to ground during function.

That's basically it!! Functions perfectly...As far as power requirements the 20 volt AC from the transformer is plenty enough for the stern board. The tricky part is integrating the trigger source in other games. When do you want it to shake and what will activate it? it is very possible to make it work however you want using standard relays and adjustable time delay relays. The PIG adapter may help in this area also. I recommend the ELK brand relays as they are solid and very functional. http://www.at-fairfax.com/ConstructionElectronics.htm

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Im not an expert but I might be able to help if anyone has specific questions.

My Pleasure

What does the relay do exactly and how do you wire it?

A relay is a smaller circuit used to control a separate but usually larger or more powerful circuit. When you turn on your home thermostat a small maybe 24 volt relay activates and switches on your 240 volt compressor outside. You would not want your compressor directly interfaced with your small thermostat for many electrical reasons. You can hear relays clicking on and off with the lighting effects on some older solid state pinball machines.

Since the shaker motor is running on high amp AC power and then rectified into DC we want to keep that separate from anything else that our machine is doing...especially since this is an add on mod and different manufacture item.

So looking at the relay diagram below lets walk through the hook up:
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The ELK-924 is actually a dual relay so you could in theory hook up 2 switched devices. This relay requires 12 or 24 volts to operate so we would wire these connections to the "power terminals". I tapped the 12 volt test terminal on my power driver board as well as the (-) ground test terminal.

The "Trigger terminal" is where you would connect what ever you want to make the shaker, lights, bells, horns, strobes, etc. activate. You will have to determine if the source is a (-) or (+) output and connect accordingly to the trigger terminal. You should use a DMM to determine the polarity of your trigger source because very often pinball devices will show (+) on both wire connections and then one will switch to a ground to activate that device. You may have to use a diode to block the voltage from going into your trigger wire so that it will function as a (-) trigger.

Using Relay "A" and since our shaker after being connect to a (+) current power source now requires a (-) ground to activate, we would first find a good ground. You could theoretically use the same ground as your relay or better a nearby ground braid. Tap the ground and then wire it into the "com"(common) terminal on the relay. Next connect the ground (brown wire) from the shaker board into the "N/O" (normally open) terminal. Normally open means this terminal stays open or disconnected until the relay is activated. Once activated the relay completes the circuit and thus activates whatever device is connected to it. The "N/C"(normally closed) terminal operates in complete opposite of the N/O side. So for example..If you had a light that you wanted to stay lit until the relay triggered, you would connect it to the N/C side and it would go out when the relay activates because the N/C circuit would disconnect and become "open". Get it?!?

Note: I recommend placing a 1-2 amp fuse on the power wire to your relay just to add extra protection to the relay device.

@ TaylorVA the parts list is above. I only used a standard Stern shaker motor that comes with its own circuit board, the ELK-924 relay, and a 2 amp fuse.

@hocusiocus using relays and now specially made interfaces such as the T.I.L.T. boards:

https://pinside.com/pinball/forum/topic/tilt-mods-deluxe-bring-your-mods-to-life

it's pretty easy to add anything to a machine now. The question is like you said "when" do I want the feature to activate. I think shakers work good on a periodic bases...just enough to add some kick to the game during a special occurrence but it has to fit the machine also!