Alright i own a 2005 monte carlo ls i have subwoofers, a viper alarm system,LED headlights, and i just had a transmission installed 2 days ago when i dropped the car off everything worked fine when i picked up my car everything exept my lowbeam headlights worked so i opened the hood to see if i couldnt figure it out i couldnt so i closed the hood and the passenger side came on so i took out my led in the driver side and put in the oem headlight and it worked once i got home and parked it for probably an hour then i started it up and they werent working now this morning they were working and stopped they havent worked since please help

 

FIRST, Welcome to MCF.
Check the little connectors inside the harnesses where they connect to the headlight housing. Look for a puke green, oxidation, or rust on both the male and female end of the connectors.
It could Also be a wire that was pinched when your trans was put in. But check your connector before you go back to the trans shop. Just to be sure!