The way it works is by intercepting the user input signal from the stalk on the steering column, BEFORE the signal gets to the control module
Originally I read the TapTurn manufacturer's instructions quickly and in my brain I thought they were talking about a DPDT switch, so you could choose to feed two wires from the stalk to either the BIU or to the TapTurn. But after reading the instructions more carefully, the TapTurn seems to detect Subaru one-touch signalling, and will slave to it if it is detected. But by cutting the wires, the TapTurn will not detect the Subaru one-touch signalling, and it will go ahead and blink the lights according to the settings the user selects. Reconnecting the wires will again allow the TapTurn to detect Subaru one-touch signalling.
I don't know whether or not the Forester harness is similar enough to the Crosstrek harness to follow the example on the TapTurn Subaru support page. I did pay for a 3-day subscription to the service manual for my 2007 Forester long ago, but haven't done that for my 2018.
I don't want to cause any warranty problems for the electrical system by cutting wires. But if I ask the dealer to change some settings, and they ask why I want those settings changed and I tell them I have an aftermarket blinker module, they may record the aftermarket part in my car's record, which may also cause extra scrutiny if there is an electrical problem.
I see that I can turn the TapTurn one-touch turn signal OFF, so I can go to the dealer and tell them to change the setting, and when they test it afterwards there should be no one-touch signalling happening. But if they press a button or move a stalk and activate some TapTurn feature that is different from stock, like the arrival/departure lighting comes on, they may clue in that I have an aftermarket blinker module. And aftermarket LED bulbs.
I'm going to email my local dealership that I bought the car from (I bought the car in April 2018), and say that there was a setting I wanted changed that we forgot to ask for when I picked the car up new, and ask if they can make the change now. I'll ask if the Subaru Select Module is a tablet thing that they can just bring out to the parking lot and use, or if they have to take the car back into the shop. Hopefully they can just send a tech out and make the change in the parking lot without any documentation being created.
Has anyone ever seen the Subaru Select Module? Is it just a little tablet thing that they can carry around?
EDIT: I guess I could put the stock blinker module back in and then go to the dealer and ask for the setting to be changed. If I did that, the only modification the dealer might see is that I have LED bulbs installed everywhere. Hopefully LED bulbs wouldn't cause any warranty problems. And swapping the blinker modules back and forth seems like unnecessary work because I don't think a blinker module would cause any damage to the car so it shouldn't cause any warranty issues.