Brake lights work but dont when tail lights are on

Have a 88 XJ. The brake lights work fine during daylight. When I turn on the headlights/tail lights the left hand brake light does not come on when I push down the brake pedal. Right hand brake light comes on.

Any ideas?

The XJ does have a trailer wiring kit installed on the L/H side that Ive stowed behind the interior panel.

I'd look for a bad ground or other bad connection. It's pretty common when there's a short or a corroded connection for the current to seek ground through the wrong element. Make sure that the lights are lighting the correct elements of the dual bulb. Brake should be the short one, taillights the longer one. Also, don't trust the appearance of the bulb itself. Try more than one.

Yah what he said.

Disconnect the trailer light adapter and re-test. Could easily be the internals in the adapter box.

winterbeater said:

Disconnect the trailer light adapter and re-test. Could easily be the internals in the adapter box.

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x2. Did the lights ever work correctly after the trailer kit was installed?