Richard Hamilton

It is always a nightmare working with non-standard wiring, and it is highly unusual (this side of the pond anyway) for a Porsche dealer to do wiring changes like that.  I think you have the right idea to check all the connections and make sure everything is secure.  Heaven knows what you will find under all that insulation tape.....
 

That looks like the control box for the seat memory.

I sent you a PM with a wiring diagram.

An "above level" fault code might indicate that the resistance of an airbag unit is high, so check the connections.  A bad contact will give you a higher resistance.  The control unit uses the resistance of each airbag or belt tensioner unit to check the system.  On a PIWIS you can read the resistance in Actual Values, so maybe check that out on Durametric.  They should be around 2.5 ohms.

Once you have any air leaks fixed, check the MAF sensor.  That is a common fault for lean or rich mixture limits reached on both banks.

With the engine idling, the MAF output should read about 14kg/hour, and increase progressively as the revs are increased.

There is only one amplifier.  I think you are at the stage where you need to get a diagnostic check by someone with a Porsche tester. I've heard good things about Porschtek in Leeds. Strasse have a good reputation too.

I agree.  It sounds like the amp has failed.  The amplifier is a separate unit, located in the front luggage compartment.  The most economical way is to get a second hand amp.  You'll find them on eBay, or try a Porsche dismantler like 9Apart.co.uk.  Another good source is boxsterbits.co.uk who often have them in stock.

I haven't had any experience with fitting an aftermarket remote system, but I can't see any reason why you couldn't connect one to the alarm module.  If you connect to the same terminals as the door lock key microswitches it should work.

Holy thread resurrection!  What mods did you have in mind?

It sounds very much like a tracker unit to me.  If you are unaware of a tracker subscription, and if it is connected to a power supply I would remove it if I were you, as they can often be the cause of battery draining.

On RoW RHD cars the seat belt buckles are NOT connected to the airbag module, they are only connected to the instrument cluster.  There are separate wiring diagrams for RHD and LHD airbag modules.

When PIWIS says "driver's side" on a RHD car, it means driver's (RH) side.

Drop me a PM with your email address. 

According to PET, the Porsche number for a 1998 RHD RH (Driver's) door lock is 8N2 837 016 B.  If you Google that number, you will find it is the same as a variant of the Audi TT. 

Look at the bottom of the page again, and you will see the user and install manuals there.....

Look lower down on the page I linked to.  You will see the black plastic housing there.  The main box is probably tucked somewhere behind the glove box.  It think you need to start from the basics.  Check the fuses, then check the MOST loop, and that you are getting power to each unit (phone module, amplifier, etc.).

This is it: https://www.dension.com/automotive/products/car-multimedia-solutions/gateway-500

The Dension is a device for connecting an iPod to the fibre optic loop.  It emulates the CDC, so that's why it has been bridged out of the loop.  There should be two orange optical cables running to the Dension, usually in a black plug.  Is it connected?

If the CD Changer isn't getting power, it will cause a break in the fibre optic loop, and there will be no sound and no phone.  First check fuses D8, D9, D10, E5 and E8 on the panel in the driver;s footwell.  If that doesn't help, check you are getting 12v to the power plug on the CDC witha multimeter or test bulb.  If you get 12v there, it may be that the glass fuse on the circuit board of the CDC has blown.  It is replaceable, but you have to take the CDC out and take off the covers to access it.

Yes, I have used it with PST2, PIWIS1 and PIWIS2, albeit only once with PIWIS2.  Personally, I prefer to use PST2 or PIWIS1 with the 993, as the software is Bosch based.

I believe these are the ones made for Durametric:  https://www.sensolutions.com/index.php?id=5&tx_commerce_pi1[showUid]=18&tx_commerce_pi1[catUid]=40&cHash=e7288b0fe8dc81c63aa62d8698735149 

I have one, and it works fine. 

If the digital speedometer is working it must be getting a speed signal, which is generated by the ABS sensors.  I would suspect the gauge itself.  If you know someone with a PST2 or PIWIS tester, they can check the analog gauge with the Drive Links function in the Instrument Cluster control module.

PIWIS 3 is the latest diagnostic unit, and as Loren has said, you need deep pockets.

My guess would be the multifunction switch on the side of the gearbox is playing up.  It is fairly common to get bad contacts in the switch.  Try moving the selector from park to drive several times fairly vigorously, to see if cleans up the contacts a bit.

Because the green light is coming on in the instrument cluster, I think you can assume that the cruise actuator is getting power from fuse B7 (black wire).  There is another black wire which goes from bridge plug BS14/2 (on the relay panel) to the clutch switch.  Could this be the black wire perhaps?  This diagram might help with checking the wiring.

Is it a Tiptronic?  If so, it might be the vacuum line which goes between the throttle body and the valve for the ATF heat exchanger.

I don't know of it is available in the US, but a lot of people swear by Fabsil Gold proofer over here.