It is not difficult to replace at all, just remove the wiper arms and the plastic cowling and there it is in the middle.
The eBay add showed several views, and I was particularly concerned with the connector - it looked right, and was.
Like I said though, this was an older model alarm that doesn't have the 'chirp' feature for locking and unlocking.
I am sure, that even thought it looks new, it is most likely several years old and the battery life is unknown.