Basically, there's relay (multi-remote control relay) in the trunk which flashes the corner lights when the alarm is turned on. We're going to buy a cheap $3 buzzer, another $4 relay, and attach it to this MCR relay so that the car beeps. Hear it for yourself:
buzzer.wav

The mod should take less than 1 hour to perform. Here's the schematic:

1. Goto Radio Shack and buy item # 273-060 (buzzer) and item # 275-248A (mini-relay).

2. Remove the Lamp and BCM fuses before starting work. The Lamp fuze is located inside the car. The BCM fuse is located in the engine bay.

3. Locate the Multi-Remote Control Relay in the trunk. Start by removing the plastic cover above the trunk latch. The relay is the brown rectangle in the picture below:

4. Remove the MCR relay and it's mounting bracket. There's a mounting tab for another relay so I used this space to mount the new relay and buzzer. I also used connectors to make the mod removable (although this could move the budget into the $10-$15 range).

5. Splice into the "Green/Orange" wire (orange is the strip color) for power (+12v). There's 3 of these wires so just choose one. Next, splice into the orange wire (solid color) for the sink from the BCM (ground).

6. Connect and test (refer to the schematic for details). The final setup looks like the picture below. Turn on the alarm and it should beep twice. Note: don't splice your buzzer into the G/B or G/Y wires (to save on the extra relay) unless you want to beep while turning...

Credit: Rob