my rt rear turn signal sometimes stop working and makes the lt brake light blink.

 

sorry its a 04 xjr

 

Possible bad earth

 

As mentioned above, when other lamps blink at the same time it is generally an earthing issue, pop the rear cover off inside the boot ( trunk ) and see if there is anything obviously corroded, may be possible to undo a nut and clean it up and refit it.

 

Ah, it's not quite that simple because the brake lights and the turn indicators are switched in their earth return wires by the Rear Electronic Module. You should check the earths (multiple) from the REM to it's earth/ground points on G24. That is in the boot on the RHS furthest forward (probably behind the RH boot liner).