Automatic Headlight Circuit on Scion FR-S

The Scion FR-S, unlike the Japanese counterpart Toyota 86 or even the Subaru sibling BRZ, does not have an optional automatic headlight circuit in the first introductory year, 2013 models.
However, this doesn't mean that you can't have it.  Here, I will show you how to add it using a simple aftermarket photo-trigger system, rigged in to the factory light switch and make it all work.

What you need basically is the following to make a nice clean install that replicates the original equipment when you are all done.

1. A generic photo-trigger system like DEI's model 545T here: http://www.deiproducts.com/servlet/the-52/DEI-545T-Nite-Lite/Detail
2. A factory Subaru BRZ turn signal stalk with foglamp and automatic headlight switch to replace the Scion part. (Make sure you get the LEFT HAND DRIVE model from USA dealers, as Japanese RHD models have the turn signals lever and wiper stalk flipped backwards and will not fit)
3. An electrical connector pin, common to modern Subaru, Honda, or Toyota small plug harness. (to make an extra wire to tap in to the blank pin socket.)

With these 3 things and a little experience with electrical connections, you should be fine.

Caution: With any electrical or fuel system service, DISCONNECT THE BATTERY so that you are safe from line-shorts and other hazard/mishaps.

Step 1: Take all of the un-necessary wires from the DEI 545T off as you will only need the 4 pins, to tap into the factory wires, and we will not even need the aftermarket relay on this install.
You should have on the main 7 pin socket, just the yellow, black, and red.  And on the relay socket just the blue trigger wire.

Step 2: Wire these 4 wires into the factory harness as follows.

The Red wire should go to a constant 12V (+)voltage source at the ignition switch or anywhere else you can readily tap nearby.  There is not a full headlight ampere load on this and is only to power the small box so any outlet is fine, that is properly fused and not critical to the main operation of the engine or the vehicle dynamics)

The Yellow should wire to the ignition trigger circuit 12V (+)

The Black wire is what turns the whole box on and off from the switch, by closing the negative side circuit of the unit, so this has to go to the pin on the headlight switch that has the ground output at AUTO position.
This is because most modern Japanese car's sub-systems are all triggered and switched via negative ground side, and the AUTO mode on the headlight is no exception.
If you look into the factory 20 Pin connector for the above mentioned AUTO/FOG switched stalk, the necessary pin connection here is #19.   However on a 2013 Scion FR-S, since the feature is not optional, you will notice there is no wire or pin in this slot #19.   So you need to take the extra pin you bought (or stole from another Denso socket) and insert it to the Pin 19.

This is the pin that has the fatter lighter blue 16 gauge blue wire in the following photo.

After connecting the Pin #19, the last wire, is the Blue wire from the DEI 545T, the ground trigger wire.  This needs to go to the pin #20 (where you see I tapped on the white with silver dot, factory wire.)
When the unit senses low light condition, this wire feeds negative ground to the main headlight switch, and activates all headlamp circuits automatically with key ignition ON.

Step 3:  Take the Photo-Cell sensor and attach it anywhere it can see daylight, and becomes dark at night.  We've chosen the factory location at the base of the front window, and on the defrost vent panel.  The DEI photo cell fits snug and neat, and by drilling a hole for the wire, no wires need be exposed either for a clean finish.  However, it does require removal of the dash cluster and other parts to fish the wire to below the dash, so that is up to you.

Plug the Photo-Cell into the DEI 545T and all factory connections back in and basically, you are done.
You can check on the system by putting it in AUTO mode, and ignition on...  And if in the day, covering the Photo-Cell should trigger the headlight on within a few seconds.   If in the night, it should turn on by itself, and you can shine a bright light like a iPhone strobe directly into the cell, and system should trigger the headlight OFF.

Well, I hope this was clear enough...  🙂

BTW: the white pin #4 on the factory socket is the FOG LAMP trigger, and it is a negative trigger.  With the DEI 545T install, you should have a pair of nice standard BOSCH relays left over, that you can keep for the time when you want to install fog lamps to this switch as well~!!