You don't need to buy both sensors, you can get the longnitudinal or lateral sensors separately.... As the sensors are exactly the same you can buy the one which doesn't come with the extra sensor attached on the bracket. One is mounted laterally and one longitudinally, one is mounted behind the glove box and the other under the steering wheel (I can't remember which is which, but if you do a search you'll find out).
You need to either remove the glove box which is very simple, or remove the cowling under the steering wheel (also very easy). Behind the glove box it's toward the upper left of the area as you look at it, under the steering wheel it's mounted on a bracket next to the column.
It's held in place by a metal band that goes round it, you just need to gently lever this off. Both side can be done it situ, but be prepared to have scrapped knuckles under the steering wheel.
As you have an "implausible signal" fault, you may just find unplugging and re-plugging the connector back in may fix it, worked for me a few months back.
