What engine in the car is irrelevant. What the car actually is would be more helpfull as the lights have nothing to do with the engine or management at all.

If it's an Escort, try the contacts at the rear light clusters as they used to give problems. Also check the earths as they used to give a lot of problems too. Side lights on one side is usually a power supply issue so if the fuses are ok, check that there is power to the fuse.