I have a bike with 4400 Tiagras, and 5500 105s, and I'm trying to find the right wire harness for them. So far I've ended up with wire harnesses that people will say will work, but neither seem compatible. One only has the sensor and a small circuit board with two buttons, the other has 2 plastic plates but the buttons are both on the right side, neither of which are useful to me. What wire harness do I need to find that has the 2 plastic plates, with one button on each side?

That doses't tell us much, you need to be real specific if asking this type of question, what are you STI you are using, rather than just saying what the bike has as a whole.

Have you tried going to the source, rather than going with what people say will work? https://techdocs.shimano.com/media/te...9830686508.pdf

Being as what you are looking at is old, think 13 years since introduction, and 7 since they were discontinued, getting the correct ones unless NOS may be an issue.

I'm pretty sure the OP is saying he has Tiagra 4400 and 105 5500 model STIs that he want to make compatible with a Flightdeck cyclometer.

I agree that the whole concept seems to have disappeared (Campy dropped their similar system years ago too) and I'd be surprised if Shimano still has any parts to support them. They weren't that common even when they were new.