As you probably suspect, all the lights you mention are supplied with current through fuse #29.

Here is the schematic if you are interested:



Component key:



10 Lights switch on dashboard between steering wheel and driver's door.

13 Parking lights in the front light cluster.

14 Rear lights in the rear light cluster.

15 Number plate lighting on the tailgate.

16 Rheostat, lighting for instruments and controls, on dashboard between the steering wheel and driver's door.

22a Fuse board behind the access panel in the glove box.

27 Direction indicator lamps, left-hand side, in front and rear light clusters.

28 Direction indicator lamps, right-hand side, in front and rear light clusters.

30 Brake lights in rear light clusters.

32 Reversing light bulbsCD: in rear light clusters.CS: in tailgate light fitting.

33 Rear fog light(s)CD: in the rear light clusters.CS: in the tailgate light fitting.

47 Main instrument display panel in the dashboard.

47v Lights-on indicator lamp in the main instrument display panel.

159 Distribution terminal (+15 circuit), in main fuse box behind the glove box.

174 Relay, daylight driving lights, on main relay board behind the glove box.

213 Pictogram in the main instrument display panel.

228a Filament monitor, rear lamps, in the luggage compartment on the left-hand side.

234 Side marker lights in the front light clusters.

255 (H8-5)Connection for trailer lighting, in luggage compartment on the left under the rear light cluster.

418 Rear light fitting, CS, on the tailgate.




It would appear that you have a short circuit or a dead ground in a hot wire somewhere in the system.

Any number of possible locations for wiring trouble. But first look closely at:

The wiring within the rear assemblies themselves. The leads are just kind of woven through pinch points in the housing.

Also remove each lamp in every socket. Inspect the socket, clean any oxidation. Most important - be certain that the lamp base is the correct style for the associated socket.

If you have unused trailer lighting harness behind the trunk liner, check it for grounding issues.

Unfortunately the fault could be anywhere and this circuit has a large number of components (spread out all over the car). It might take some time to locate the problem.

 

So I recon the best way would be to cut out the seperate parts leading to the fuse and check each part whether it is connected to the mass? (check the resistance,1=ok, 0,x= the wiring touches the mass)

If this is not the case, I would have some component that is broken, right? Maybe a relay that has a shortcircuit or creating surges in power?

I'm not a mechanic but I do have some elementary knowledge of power systems. Please point out if I'm missing anything.

 

Below is an excerpt from a key-off drain diagnostic that I wrote a few years ago. You should be able to adapt the procedure find your dead ground/short.

But first, to protect the wiring harness, you might want to buy a bunch of fuses instead of making a test lamp apparatus.

Just using the fuse as an indicator (much safer):

Unplug everything from the circuit (at the component end of the harness) and install a new fuse.

If the fuse immediately blows - the problem is in the wiring harness.

If it hangs in there, start reconnecting individual loads one by one. Check the fuse after each plug-in. When the fuse pops - you have located the faulty branch of the circuit. The problem lies within the last component to be connected. Inspect and repair as necessary. This is why you bought extra fuses - it may take several attempts as you experiment with the wiring (looking for the exact point of the failure).



Using a test lamp as an indicator (not for the faint of heart):

If you do decide to use a test lamp across the fuse holder, be ready to immediately kill the power (i.e. yank the test leads out of the fuse block) if the lamp goes really bright.


Make a 12-volt test light. Take a couple of tiny male spade type solderless connections (Stakons) and crimp one to each of the two test lamp leads. BTW - this procedure will be much easier if you use nice long leads for the test lamp. That way you can keep the lamp right near you as you work (more on that later).

Plug these into the fuse holder just as if they were the prongs of a fuse. Make it up well and secure it tightly so it can’t get yanked out as you move around.


Now look in your owners manual (sometimes it will give more detail than the fuse box cover) and see what things are driven by this circuit.

The lamp should be out or very dim with everything unplugged. Then, while dragging your test lamp around with you (see why I said to make it up with nice long leads) start re-connecting things one at a time.

Watch the lamp at all times. When you connect the faulty group to the circuit, the test lamp will dramatically increase in brightness. YANK IT OUT OF THE FUSE BLOCK FAST.


You have found the bad component group.

Repair it or replace as necessary. Plug everything else back in too. Check your work with the lamp.

Okay, what if you disconnected everything single thing in the circuit and the lamp is still bright (or a new fuse pops immediately)?
Either your owners manual is not providing enough detail and there are other things still connected to the circuit or, and this sucks- it is in the wiring harness.

Pay special attention in areas where the wiring can touch the frame or where it passes through a tight penetration.

The fault will almost always be found in the insulation of the wires (chafed through) or the lamp sockets (e.g twisted/bent contact springs touching each other).

Good luck. There are other methods to find a buried ground (the smoke test for example) but they are for professionals only. But with any luck, these methods should find and repair your ground.

 

Thanks a lot!

I'll start on this tommorrow probably.