I fixed my 300M's center brake light, but ...

Is the car in locked mode (alarm-armed) when you open and close the trunk and the alarm goes off?

Or is the car in unlocked mode - and the alarm goes off when you use the key-fob (or the dashboard-mounted button) to open the trunk?

Here's your problem:

With the VTSS (Vehicle Theft Security System) ARMED, opening the trunk (Deck Lid) triggers an alarm.

The deck lid has a Knock Out Switch that lets the BCM (Body control module) know if the lock cylinder is either in-place or has been knocked out.

If the cylinder has been ?knocked out? or (as I believe in your case) the wiring to the switch has been damaged, when the lid is opened and the Deck Lid Release Solenoid/Ajar Switch sends it?s ?Ajar? signal to the BCM the VTSS will trigger the alarm.

I would guess that you have a wire break in the deck lid?s wiring harness.

There are several wires in that harness (release solenoid, high mounted stop lamps, grounds, etc.) and I suspect that you have a problem with the power feed to the ?knock out? switch.

Check the harness where it transitions from the ?deck lid? proper to the interior of the trunk. That?s where you'll find any wire breaks. The harness flexes a lot there and over time something can give.