Rick150

There is no direct wiring from the drivers door to the tailgate, it goes to the Driver identification control unit that decides if all the conditions are met to open or close it then passes it on. As you can see from the circuits of the Tailgate that it all terminates to the CAN BUS which talks to the control unit. Could be that one of the conditions to activate is not true, which one I have no idea.

The Cayenne has an electrical system that relies on LOGIC that's present on the CAN lines and in the control unit there will be a 'Truth Table' that decides if to let you activate anything. If you had a scanner it might point to what seems like an unrelated fault code that could help.

The wiring for the lights is quite extensive and quite difficult to put into a format to post. I recommend going to eBay and purchasing a manual.  Not all models comply with the wiring diagrams I have as they are mostly for the 957 although 955 is pretty much the same.

This shows the lock and light combination.

Just to put my mine at rest did you check fuse 31? when you try and open the tailgate it will make noise but not open if 31 is blown. It's under the left side dash cover.

Don't know much about the trailer hitch, only that it is coded to the vehicle.

I would check Ground MB41 near the left tail light.

My workshop manual tells me they are on the passanger side with acess through the glove box.

posted it again, should be a total of 8 pics.

Trying to post the procedure but exceeded the total file size, sorry

This may help

Brown wires from these connect to MB23

Your correct, MB23 is an important Ground for all those systems.

Hope this helps. Do as JPF says, check the resistance.

Check the high current connections, they are to thick ones from the battery under the seat to the starting circuit and to ground.

Now that you know it's a corrosion problem then unbolt and clean them, don't just look. Check any connectors you can get access to as well.

Battery connections to the starter is where you feel the heat.

Clear the suspension warning, in fact all warnings now you have fixed most of your problems and see if they re-appear.

Check the CCA of the starter battery, better still plug in a diagnostic unit that has the voltage curve when starting.

After starting the car put your hand on the terminals related to starting and if they are warm you have a bonding problem, can be + or -

Here is what you requested.

There will be error codes, what are they?

mb23 is ground point, you can see the brown wires going to it.

This is info you need for the charge relay.

First of all get the Crank Current (CC) of your battery checked because voltage doesn't mean much, you need to know the health of the battery.

Check MB23 as you have already checked GP25? Check the fuses.

The alternator must have windings excited to make power so it could be on the regulator side of the system with a problem with low voltage, Bad ground?

Here is the Charging Circuit.

MIG welding is recommended.

I would buy a genuine stud but it's up to you.

Here is an option that will save you from welding but it has to be done right!

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Remember that wiring corrodes on the Porsche because it has a galvanised body and when two dissimilar metals have a current path like moisture or even high humidity air the wires get eaten away. So, if you fit anything to a Porsche body make sure you use a paste to stop this action.

This is from the body repair instructions.

They are welded, need to grind it off from below, drill 8mm hole, fit the replacement by welding it underneath to the body. Stud should be M6, normally that would be 10mm socket for the nut. Tightening torque is 6.5ftlb.

When wet is a clue, look at the brake pad sensors including any speed sensors the ABS uses for each wheel. Wet is the direction it's telling you.

Electrical problems can be elusive. Do one repair at a time, take note of what's changed then as you fix them you will gain knowledge and hunches as to what the problems are. I have no idea if that starting problem is related.

Brown is true ground but black is used all over the wiring, sometimes used as reference for a control circuit, sometimes it says could be BLK or BRN but I see some black wires have voltage on them. If the wires are Black then they are being used for ground but don't treat other Black wires in the car as ground. If you have this problem then look at the other bonding points, get ahead of the game.

I sent you the Ground points, which one is it, GP25?

Place an OHM meter from the wire to the stud, there should be less than 2 OHMS. Unbolt the Ground nut and clean with  a 3M pad or if you don't have one use pot scouring plastic pad, don't use sand paper as it leaves small practicals embedded in the soft copper or tin.

Get a manual from www.usomanutenzione.it for your model Cayenne. I will continue to look through about the 12,500 pages and I don't keep a record of what I sent you.

Most likely a connector or Ground issue.

Could be 'ground' problems, terminal MB40 for the Rear Comfort Systems Control Unit is near the right tail light which includes Lid actuator etc. There are many brown (BRN) that go to the body. Also another (MB41) near left tail light. Ground for some rear relays (MB23) on left side of dash. These ground terminations also service other electronics and it could be why your instrument light flickers.  The Cayenne multiple grounds through the galvanised welded body to provide a low resistance path, should just one of these have resistance go high the electronics can do wearied things.  Most electronics seeing anything other than a few ohms can read it as something else and get confused.

Included are 'Ground Bonding' locations to check.

'Connector locations' in case you have a corrosion issue.

'Main Wiring Harness' routing.

Here is the complete wiring for the power locks plus Lid loom.