92 gmc sonoma...when i turn the headlights on the brake lights and reverse lights will not work when i turn the headlights off they will work..the taillights are flickering but they wont stay constant

Check bulb on side that only works when headlights off.

Hope I helped,
Luke

this indicates a problem battery (have a load test done on it) , drive belt adjustment or condition or problem alternator out put
with the head lights on there is insufficient capacity from the battery and/or alternator to keep everything running as it should

Consider the dimmer switch on the steering wheel; the wires from it wouldn't affect the tail lights, but could possibly affect the headlights. Hopefully loose wires from the assy is the problem.. replacing the unit might be difficult. or expensive.

Is the dimmer turned to the light on position by chance

Duplicate question.

well try this it would make you laugh if this work check your floor brake pull up on it all the way and see if it comes on when the park brake is on they turn the lights off and if there not up all the way it will keep it from coming back on. hope that works for ya.

is there a curcit board in the rear tail light assembly if so change it

check the headlight switch for burned contacts or a heat dis-colored connector to it. If u find either replace the headlight switch and the connector as a pair.

yes , i have a 96 mine done the same thing , i was removing the dash cover to install a radio accidently broke mine , i had to push down on what remained and wedged a sheetrock screw to hold it in place till i could get a new one

please check your brake light switch located near the brake pedal. it could be faulty.

tnx 4 using fixya,

drcool