Disconnect buzzer on a 2001 gmc

Wiring diagrams.

https://www.google.com/search?q=2001+gmc+2500hd+headlights+wiring+diagram

and

https://www.google.com/search?q=2001+gmc+2500hd+headlights+wiring+pinout

Plus

https://www.alldata.com/diy-us/en

..

fix head lamps first, they work key on, or key off, simple
so a lose fuse? for HL
got DLR lamps in USA truck> daylight running lamps.
HD tailer hitch option?
head lamps Low and high dead or both and flash highs fails ???????
park lamps work? front and rear?
4wd out means 4wd does not work or only the silly 4wd lamp.
???????????????
yes park lamps are related to interior , back ground lamps (dimmed too) and lots of relays and BCM for all that.
check for more lamps dead inside and outside. good and bad
the truck came in 3 trim levels and lots of options added for all this.
even auto dim etc.

The daytime lights run off the highbeam circuit on reduced voltage. If the lowbeam is not operational, take a look at the plug at the headlight and inspect for looseness. If tight, inspect for a constant ground and a 12v power supply with the headlights turned on low beam at the headlight connector. repair a necessary

My 2010 tacoma doesn't have a buzzer. The lights automatically go off when you turn off the engine and open a door.

Check for power at the two headlamp fuses. If there is no power than try swapping the headlight relay. Let me know what you find

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

It can be issue with headlight switch,but as its already been replaced.Try disconnecting high beam headlight wiring from headlight assembly and reconnect direct external wire from headlight switch to headlight assembly,then by switch see if headlight is getting off and turning on or not.
See below related help links,similar to your issue.Click the link below:----
Only high beams work but head lights not working on Ford F350?

http://howtobyme.blogspot.com/2011/11/only-high-beams-work-but-head-lights.html

----------------
Only high beams working on GMC Savana?---------- High beams stopped working but headlights still work? http://howtobyme.blogspot.com/2011/11/highbeams-stopped-working-but.html
---------- • Working of Headlight switch? ----------

• Both headlight bulbs blow together? ----------

• Car headlight bulbs don't last long?
------------ Only one head light works on 2001 Dodge Caravan? http://howtobyme.blogspot.com/2011/11/only-one-head-light-works-on-2001-dodge.html

----------
these will help.
Thanks.

Check the headlight switch.

Believe it or not but I replaced the heater resistor switch to solve an a/c problem and now the buzzer only sounds when it is supposed to. Go figure...

Have you checked that the headlight switch is in the right position? Are the headlights or parking lights still on when the key is removed?

If so, then rotate the lamp switch to the correct position and the buzzer will go off. Headlights and parking lights are not controlled through the key circuits.