Car alarms are, in general, a waste of money. They have too many false alarms, are ignored by everyone because they are so common, and can annoy the heck out of your neighbors. The best thing to do is make your radios removable, either by using portables or designing the mount so the radio has a quick connect and is mounted on a slide. If the radios are in the vehicle, a thief will get them no matter what kind of alarm you have. The best thing you can do is always lock the doors and hope for the best.
One of our unmarked units was broken into while thee deputies were at lunch in a major shopping center in broad daylight. They got two mobile radios and a scanner. They almost got the shotgun as well but didn't have a big enough pry bar to break it free from the mount. These guys knew it was apolice car and broke in anyway. Your chances in a civilian vehicle are zero if the radios are visible and the the thief knows what he's doing.