I remember Porny, the td5 ecu guru from here, saying they can cause some problems due to changing the resistance on some sensitive circuits....
His suggestion, for a Defender, was to move it onto the bulkhead behind the centre seat / cubby box.
If you open the hatch you will see that the cables route from under the centre over to the seat box - so they are plenty long enough.
You'll need to drill two 60 mm holes and a 32 mm one (I think!) and you can then re-use the grommets. The 60mm is for the ECU block connector and the 32mm is for the fuse box.
I'm going to be doing this soon.