I have the lockable Touratech mount. It's expensive but really good. I got myself a crossbar on my 12GS and mounted it straight to it (you can also use a ram mount to attach it, but as the ram mounts are "quick release" the locking part is rendered completely useless). I wouldn't trust locking it up overnight in a public place, but it's enough to deter thieves at petrol stations etc.
Cable-wise, You'll need power/audio leads for it. Are you plugging it into an autocom system? If so you'll need the audio cable for that (I think it's No. 1294 off the top of my head). There's a Garmin lead for bikes that does both power and audio which you'll need anyway I should think.
