Tail lights not working, bell for lights left on or key in ignition sounds when exiting vehicle.

Power flows from the 12 volt battery through Fuse 19 (20A) and to the Tail Light Relay. This fuse and relay are housed in the under-hood fuse/relay box located at the left rear of the engine compartment. When the Tail Light Relay is commanded ON by the MICU, power flows out from the relay to Fuse 27 (10A) housed in the under-dash fuse/relay box located at the lower left end of the dashboard. From here, power flows to the side marker lights, license plate lights and tail lights. If a fuse is blown, check for water in the tail light assemblies. If a fuse isn't blown, it could be the the Tail Light Relay has failed or is not being commanded ON by the MICU. The MICU wouldn't send a command to turn ON without first seeing the request from the headlight/combination switch, so there could potentially be an issue there as well. You can read more here: https://www.yourmechanic.com/article/symptoms-of-a-bad-or-failing-combination-switch-assembly

Sounds like you may have a short within the brake light switch itself. I'm not familiar with the vehicle but this electric switch will likely be located under the dash on the brake pedal itself.

loose wire or maybe a one thats ben cut , shorting out something

The dashboard module can also cause this, but it is a good possibility that the bcm is bad. Check for water intrusion around the bcm and under the passenger side of the dash. The drain tube for the a/c can get clogged with debris and cause the water runoff to come back into vehicle and create many problems.

Yep through the dimmer switch itself. The dimmer switch is either bad or the pigatail to the dimmer switch is bad.

Good news. After letting the van sit for a few hours, when I went to get in it again, I opened the van with the key (ie. not the remote fob) and there was no beeping. I don't know if checking all the fuses (I fulled and checked each one), or disconnecting the battery, using the key, or just letting it sit did the trick, but it is now fixed.

test lite the bulb, sounds like the bulb iz bad mebe,do u have trailer lites hooked up to tail lites? could be the wire harness. let me know.

Check the TAILGHT FUSE. GM's unusual way of letting you know there is something wrong. Otherwise how often do you check your tailight operation. Only by another driver telling you, your tailights aren't working, would you know. Hope this helps.

check to see if water is entering your tailight housing some how,It may be that water or condensation is entering the left or right tail light,which is causing the bell & check your break light bubs

The bell that warns of keys in ignition is weak sounding in my truck also, but all my lights work. By weak i meen harldy audible at all. My conclusion is that they are not on the same circuit.