What could be the issue for the cluster to not buzz to an open door?

It sounds like the key warning switch inside the ignition key tumbler has gone bad, I would check all fuses and then switch the ignition lock out for a new unit. Check out the images (below). Please let us know what happens.

The old owner said he replaced the key cylinder. What wire enables the buzzer? Maybe it's the seat, because under the seat its disconnected, causing it to not buzz. My 06 Scion xB does this when the seatbelt buzzer is disconnected.

Yep, that could do it, can you connect the wiring under the driver seat to see if it fixed the problem?

It's because i am missing the buzzer relay that plugs into the fuse box, just like the blinker relay.

Okay, that makes sense, have you replaced the relay, does the buzzer work now?

No i have not replaced it. I was researching why it was not and apparently most VWs like Passat's, my Corrado, Jetta's and Golf have this buzzer Relay, of course on older models. But once you plug it in you should have it working.

Sounds good, I am glad you got the problem figured out.