I have a lexus ls400 and my tail lights don't come on. They work when I press the brakes. Is it a fuse or something? I already replaced the bulbs.

Its you taillight relay located in trunk on driver side directly in corner mounted next to the stereo amp

Check wiring harness that runs round boot/trunk hinge.these get stretched and cause all sorts of probs

1. I don't have the Year of Your Vehicle Owners Manual will tell You where Fuses are Located, if not I have Resources to Find it for you, need year of Vehicle.

• Headlight switch is Bad because there is a park light lead for Front and another for the Rear Coming out or the Switch.CHECK THIS "AFTER" Rueling OUT Fuse and Cause below.
• Is A Bad GROUND Connection in the Trunk that Runs From A Connector or Screwed to the Steal of trunk So Fallow the Light Harness FROM Tail Lamps Towards front of trunk Look for A Connector or Ground to BODY in Trunk see if it is Corroded if so Repair With New Eye Connector and Clean to BARE Metal Where its Mounted. IF YOU Find a Harness CONNECTOR Unplug it and inspect it for Corrosion if it is Clean With Electrical Spray Cleaner After this you may need to Plug & unplug a number of time to get a Good Clean Contact then Spray them out again. Plug em together check , If Harness Plugs Were Corroded and still don't work after this Plug is UNREPAIRABLE and must be replaced.
• I truly Hope this info is of value to you and it Gives you Tail Lights as THATS A TICKIC And I Love Helping folks out.
• Have A good Day, Rejakwilson

Re please and grease the inside the wire harness be for putting in bulbs.