Whilst I know it is not mandatory, I would like the peace of mind of having one... My question is which? Does it need to be a certain size? The blue book doesn't seem to specify this, but does seem to recommend the electrical version over the mechanical (3.2.7 Method of Operation: The preferred method of operation is electrical which should have its own source of energy for triggering, ideally with provision for checking the integrity of the systems triggering circuit) .
And then what are the pros and cons to mounting position - passenger footwell vs boot. I suspect that the electrical one would be easier to set up with the switch rather than the cable pull. as it needs to be next to the electrical cut off (although maybe that is simple if it is in the passenger footwell).
3.2.2 The triggering point from the exterior must be positioned close to the Circuit Breaker (or combined with it) and must be marked by the letter “E’’ in red inside a white circle of at least 10cm diameter with a red edge.
I thought I read that the systems should be mounted to prevent torpedoing - but can't see that in the blue pdf er book...