When I install alarm systems, I have found that the best place for a shock sensor is on the main ignition harness itself, you may have to adjust the sensitivity though. The harness is the best place because it can pick up vibration from the rest of the vehicle plus if someone was to get inside without triggering the alarm, the harness is usually the first place they would go, upon touching it the alarm would immediately trigger. This is what we have found to be a best practice and are trained to do. I have my shock sensor on my harness, zip-tied then wrapped so it is secured and it works great.
84 Fiero SE