Its under the seat, attached to the seat itself. By far the easiest way to get to it is to just take the seat out (undo the 4 big Torx bolts, one at each end of the 2 seat rails, then undo a connector to disconnect the seat wiring). You need to move the seat forward and back to get to all the bolts so leave the connector till last. Other than that its a no brainer and takes about a minute.

This or a very similar one was an issue on the old XK8/R (X100 series). The solution is to repair/replace the electronic part of the ABS module (Bosch/Teeves unit). What is going on is that theres a motor or solenoid (cant remember which) that is soldered directly to the circuit board, and vibrations over time break down the solder joints. It just needs re-soldering. The electronic module is sealed so if you want to do it yourself you have to cut it open along the edge, do the repair and re-close it with silicon mastic. If you don't want to do it yourself there are companies that will sell/trade you a "reconditioned" unit for yours, which works out A LOT cheaper than buying a whole new ABS unit. At least on the XK8, if you're careful you can detach and (just barely) wangle out/in the electronic module without also having to drain/detatch/re-attach/fill/bleed the brake pipes going to the ABS unit. I've done it before, its not hard just awkward. See this thread for more info:...