Battery disconnect not needed for this, but do it to be safe, and wait 15 mins before unplugging the pre tensioner.
I probably did not explain very well, fact is it could be anything the others have said, but if it is the side of the connector on the loom that comes out the floor, a new pre tensioner may not cure the fault. You have used a icarsoft, so you know more than you think, when you look at the connector, you will think, 'is that it, bloody Devilish made it sound like diffusing a bomb'.
Years of movement make the pins inside the connector not as snug as they were.
Under the front of the seat near the centre console there is a small yellow 2 pin connector, have a play with it, and see if the warning goes out. This is only diagnosis, the seat will have to come out anyway really, for the permanent cure.
If not, the seat will have to come out, remove the pre tensioner from the seat, and plug it back in several times. sometimes this works, but no guarantee.
What I do is remove the male pins from the connector (some as thin as needles) I run a soldering iron along the pins to give them the slightest coat of solder, smear dielectric grease on them. If it is loose connector pins, job done, and original connector in tact.
That is why I suggested taking the pins out the connector, that is fiddly, and just plugging the bare pins into each other.
Other option is to peel back the loom sheathing, snip off the connector, and solder on a pre wired 2 pin connector using heat shrink tube ( there are many, but I would use aTyco). I crimp my own pins like many others. If it does not work, and it is the pre tensioner, it has only cost a quid or two, you can fit the Tyco to the new pre tensioner, and voila. I must have at least a dozen pre tensioners, from upgrading seats for folk. many with the connector cut off.
Pre wired
I use these and crimp my own pins
Once the SRS warning lamp is out, re fit the pre tensioner to the seat and put it back.
If you do decide to replace the connector, you will notice the wire colours on one side of the connector look almost the same colour. Well one side has black and something else. The wire that goes to the black, wire, colour the end with a black marker, in case you forget.