To be totaly honest a theif that will go the extent of using jammers and jarjacking aint gonna nick a 20 year old sierra they are going for bentley mercs bm's etc. Some little cock that wants to take your 20 year old cossie for joy ride wouldn't care if it had a tracker or not.

Form doing car security for nearly 12 years i highly recommend the clifford black jax along with a cat 1 and a Tracker locate. thats what i've got. oh and a steering wheel lock thing(big yellow job that covers the steering wheel). The secuirty i have on my car will keep it safe from a opertunist theif/joy rider but if someone really wants it there'll get it.