I am replacing the existing thermostat with a nest thermostat E, and it gives me e195 error on the nest device. Looks like I am missing a C wire, so I checked the air handler in the attic (made by First Co, model number 18HBXB-HW), I see the brown and blue wire is not connected to anywhere on the unit, I am thinking I can just connect them to the unit to get the 24v AC, but the question now is which wires on the unite provides 24v? the manual of the air handler doesn't seem to be very clear to me. here is the link: https://www.firstco.com/documents/ProductDocuments/hbxb-hw1117.pdf
You want the brown wire
Based on the diagrams in the manual, the brown wire is the C wire you're after, so connect the blue wire in your thermostat cable to the existing bundle there. We can also tell it's your C wire from the fact the cable off to your air conditioner's outdoor unit connects to that wire in addition to the yellow Y wire from your thermostat.