When the headlights are on and the brakes are applied the right tail light goes out and the right brake light does not come on. the left brake light activates and the cyclops activates.

Stop Lamps
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The body control module (BCM) provides a 5-volt reference voltage and low reference to the brake pedal position sensor. The position sensor provides an analog voltage signal back to the BCM that will increase as the brake pedal is applied. When the brake is applied, the BCM will then apply battery voltage to the left and right stop/turn signal lamp supply voltage circuits and the center high mounted stop lamp (CHMSL) supply voltage circuit illuminating the stop lamps. At the same time the BCM applies battery voltage through the signal circuit to the transmission control module (TCM) and the engine control module (ECM) indicating that the brakes are applied.
Ground for the stop lamps is supplied as follows:
• G402 provides ground for the left rear stop/tail/turn signal lamps
• G403 provides ground for the right rear stop/tail/turn signal lamps
• G301 provides ground for the CHMSL
The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.
Battery voltage from the LT TRN/SIG and RT TRN/SIG fuses located in the rear fuse block is also used by the body control module (BCM) for stop lamp operation.

Turn Signal Lamps
Battery voltage supplied to the body control module (BCM) from the fuses listed below is used for turn signal and hazard lamp operation.
• LT TRN/SIG fuse for the left turn signals
• CTSY/RF TRN fuse for the right front turn signal
• RR TRN/SIG fuse for the right rear turn signal
When the turn signal switch is place in either the LEFT or RIGHT position, ground from G300 is applied through the turn signal switch signal circuit to the BCM indicating the turn signal request. In response to this signal, the BCM applies a pulsating voltage to the front and rear turn signal lamps supply voltage circuits cycling the lamps ON and OFF. At the same time the BCM sends a GMLAN serial data message to the instrument panel cluster (IPC) to cycle the appropriate turn signal indicator ON and OFF.
Ground for the turn signal lamps is supplied as follows:
• G104 provides ground for the left and right front turn signal lamps
• G402 provides ground for the left rear stop/tail/turn signal lamps
• G403 provides ground for the right rear stop/tail/turn signal lamps

Do you have a factory scan tool (tech2) ? Your best bet , take it to the dealer .

2004 Jeep Grand Cherokee Service Repair Workshop Manual INSTANT DOWNLOAD

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Sounds like you have a bad ground at the left rear corner , find wiring harness follow it back till you find ground clean connection and retest lights

Headlight ground is a common problem. Also, check that the battery ground is sound, if battery ground goes to engine, check that engine strap to frame is also sound.

will u have a few more switches and relays to check/change brake light switch and relay/and headlight switch

you have a bad ground. go to that light bulb socket and check for ground to the socket.

you might have an open ground wire on your brake/tail lights .. when you apply the brakes then power would flow thru the brake light (not see ground) so would then provide power backwards thru the parking light (common ground open) to all the other things connected to the parking lights .. like the other parking lights and dash ... when you turn power on to those things (headlights on) then there is 12 volts on both sides of the light .. thats why the lights quit working ... if the missing ground is the problem then most of the lights that are improperly turning on should be at a lowered brightness level.. interesting problem .. its also possible to have a brake light bulb that has the common ground open inside or not making connection with its socket .. i would start by removing the brake lamps and see if the problem goes away .. if it does look for a broken ground or damaged lamp ..

the tail light and brake lights are working from double contact bulbs. check the grounding, you dont have enough ground from one of the bulbs

I had the same problem. They were on when the headlights were on, but turned off instead of brightening when I pressed the brake. I replaced the bulb and now it gets bright when I press the brake instead of turning off (like it should).

Hi Ahunley, Sounds like you have cross the connection wires to the unit ( positive to earth & earth to positive) Try swapping the connectors over.

Hope this works.
kind regards
David