Does anyone know why my left marker lights come on when I press the brake pedal?

 

You have a short to ground or a bad ground somewhere in that circuit. Check the grounds , wiring and connectors and also the bulb sockets and bulbs.

Charlie

 

Yes, it's probably "bad ground" at the stop lamp connector or wiring. The stop lamp bulb usually has two filaments, one for the brake light and the other for the parking light, that share the ground side. When the ground wire is not connected to the ground, the brake light current will flow through both filaments in series and then through the side marker lamp. Pull the side marker bulb out and you may see the license plate bulb turn on with the brake light.