You should check out a forum specific to your car model, to determine where the wires and cables are for your car. Also, check out a Jensen specific forum for information about bypassing this safety feature on your unit.
I can, however, tell you that in general, you need to just ground the wire that goes to the Brake Lever switch. HOWEVER, you state you need the Brake Cable, which makes no sense to me, so perhaps the Jensen does things differently. YOU need to ask this question some where else.