Tail lights wont work on '99 Tahoe but brake lights do and checked bulbs and fuses already they are ok. Why won't the tail lights work when i turn on my lights?

If the fuse for tailights/parking lights is good and the bulbs are good (double filamented larger for parking lights)

check to see if you are getting power to the sockets

check to see if you are getting ground to the sockets

if so - may be a bad switch

you could go to your local auto zone and by the tal light circuit boards cause Chevy has a problem with those parts going out. the tail light and the brake light make be on the same board but they run on separate wires