The rear lights on my R320 have failed. I have fitted new bulbs but without success. Is there a fuse for these lights

no fuse open trunk and access light sockets

The fuse box will be under the hood, (bonnet), near the battery. The light fitting is usually clipped in from behind, inside the trunk, (boot). There should be a section of trunk liner that will fold out to reveal the back of the light fitting. The connecting wire should unclip and the back of the fitting should lift out.
The bulb should twist and pull out and the new one should push and twist in. Refit the connecting wire and light fitting back into place. Test the bulb is working and Refit the trunk liner.

Could be a fuse but if it is you probably did something that caused a short circuit. Fitting a wrong bulb is not unusual.

Unlikely both rear blinker bulbs would fail at the same time. It is better to work towards a diagnosis by checking the bulbs, testing for a supply and a good ground than replacing bulbs and hoping.

A little used hazard warning light switch is often the cause of sudden indicator failure or even partial failure. A couple of minutes of rapidly operating it on and off often put matters right.

You have a bad short somewhere in the circut that the fuse applies to. I'd guess it is in the taillight printed circut film, or connection and or the ground wire connecting the film to the frame.

it would be the bulb fitting. they can go weak or discoloured and not contacted the bulb fitting. take the bulb out and clean the fitting. failing that check the plug to the light fitting it can get a covering of oxidised coating on them breaking the circuit.

This is likely caused by using the wrong bulb.

You can certainly purchase after market bulbs of more modern technology that provide much better illumination than the manufacturer recommended brand. HOWEVER, those bulbs having a matching part number to what is recommended does NOT mean it is what you should necessarily use. The reason is the wattage. Higher wattage produces more heat.
Try a bulb of a lower wattage rating. I recommend going no higher than 20 watts beyond your original rating, do not allow the bulb glass to get dirty while installing nor touch with your bare fingers, and inspect your ground connections to the headlight assembly for any breakage, corrosion or looseness. Good luck!

The switch is fine if the third brake light is functioning. Even though the tail lights work, the filament for the brake light can be burned out. Sometimes they look intact but are still burned out. Replece the bulbs.

Check your fuses underneath the drivers side steering wheel. Flip the plastic cover over and there will be a diagram showing the fuse that controls the taillights/stoplamp. Pull the fuse out and replace it with the same number fuse and your taillights and brake lights will work again. If the fuses are ok, pull each taillight bulb out and replace it with a new bulb. Sometimes when one light bulb fails, all of them will fail.

I guess that your refering to the backup lights? They have their own fuse and aren't imune to having the bulbs burnt out. The bulbs are easiest to check as they may be black in color or knocked out of their holders. The back up light switch is part of the neutral start switch and they will have other problems like no start if it's bad. wiring and their plugs are also possible mainly if the car as been in an accident or other bending action.

If other parking light work in the front or the rear than you don't have a fuse problem.You need to check the wiring to that lamp or you just need a new bulb.