You probably have a rusted or disconnected ground wire to the running lights and side markers.
When the headlights are on.
Check fuses Check bulbs
It's the head lights only.
There is a shorted wire or bulb in that circuit.
Short to ground in the circuit or a faulty chassis node.
Remove from under the hood, the side marker lights have to be removed before the headlights will come out. You also have to remove the battery and the air box.
Brake lights should NOT be effected by the headlights. Brake light function is completely independent of the headlights. Assuming you are not refering to the tail lights, or marker lights, which are supposed to come when the head lights are turned on. Not having brake lights when the head lights are on is a big safety problem and should be corrected.
On my 98 sunfire, the tail lights and marker lights only work if you turn on the headlights. If only the daytime lights are on, the Pontiac on the trunk is illuminated but no other lights at the rear until brakes or turn signals are activated. This is common with a lot of vehicles out there. Daytime running lights are not necessarily the same candle power as the headlights. I can tell a big difference in mine.
Probably a blown fuse or relay, which may be the result of either age, or perhaps a short somewhere.
Turn signal lights that double as parking (marker lights) when not blinking.
command start malfunction, disconnect, problem solved.
To assist drivers in maintaining road safety, marker lights are installed in road lanes and partitions, making them easily visible. Marker lights can either be LED-powered or merely reflective.