[Mrmerlin]

since your not familiar a few pointers
The battery is in the rear of the car under the spare tire in a box.

The most common failure point is a corroded ground strap from the neg terminal of the battery to the chassis,
the strap can fail at the chassis ground,
clean this connection with an eraser, do both sides of the ground strap also look at the strap itself see if the webbing is corroded inside the plastic sheathing, if so then replace the ground strap.
Also clean the battery terminals both pos and neg take all of the connections apart and clean them .
There is a big ground strap from the right side of the Block to the chassis just under the # 3 exhaust port. Clean this on both sides.
NOTE the 928 Firing order is 1 to 4 on front right,
5 through 8 on the left front as viewed from the drivers seat.

Next is a connection called a hot post, this is on the right fender well next to the front of the timing belt cover, As it states it is hot all the time the battery is connected there should be a cap that covers the 11mm stud and a black cover that goes over the wires, (if there isnt one then get a hot post cover from a 1988 S4 )
remove the hot post stud cover and the black cover you will see an 11mm stud holding 3 thicker wires ..
(these come from the alternator and go to the CE or central electric panel this CE panel is just under the glovebox , remove the footwell carpets and lift the wooden cover to get to it.)
Unscrew the 11mm stud with the battery disconnected and clean the ends of these wires with the eraser.

Also just above the hot post stud is a 14 pin connector this connector also has power flowing when the battery is connected (Hence the need for the black hot post cover) this connector is prone to getting wet if the cover is not installed this will cause corrosion on the pins clean them with the eraser.
refit all of the connections pay attention to the thicker red wire thats connected to the hot post this wire can sometimes shed its insulation its a good idea to put a piece of heatshrink on it prior to refitting it since its carrying positive 12v all the time.
Next go to the CE panel there are 2 bolts above the panel they hold about 12 brown wires these are grounds for various systems, just above the CE panel is the bottom of the blower box, see the grate.
This blower box is also prone to leaking water onto the CE panel from its edge seal , this will be evidenced by seeing rusty bolt threads and white drip trails down the CE panel supports, if you see this then its a good chance the Blower box needs to come out for a reseal of the strip caulk.

Anyway clean the brown wire ground connections above the CE panel.

Also on top- of the center of the CE panel is a clear cover, this covers about 3 or 4 red wires this is the power supply to the CE panel from the hot post, clean these connectors as well.

Get a fuse relay chart here 928gt.com on page 2 under tips and links print off a copy.
The Fuses are numbered from 1 to 45 or possibly less on the older cars starting from the left side of the panel as you look at it.
Remove each fuse on the Ce panel and clean the ends of the fuses and the fuse holders, with the eraser.
Make sure that the correct fuse is in the correct slot, also make sure that any empty slots are in fact empty.

Next do the relays, there are specific relays for the various systems, some are common called # 53 relays .
Anyway remove the relays ans note the positions, there are Roman numerals on the CE panel with small arrows that point to the respective positions. Make sure the correct relay is in the correct slot, also make sure that the empty slots are in fact empty.
there should be a few slots that dont have any relay in them.
Clean the relay pins with the eraser, any relay with corroded pins should be smelled for burning and opened and the internals inspected, water dripping onto the CE panel will cause this damage.
. Make4 sure to verify the correct relay is in fact in the correct position , there may be a relay with a fuse tester on its top this is a #53 relay, so it can be installed in a #53 position.

Moving on check the starter connections make sure the wires are tight and the wires are not touching the sway bar,
the sway bar will wear through the harness as it moves when the car is in motion.
Next check the alternator, this may require dropping it to remove the back cover, leave the alternator hanging with a piece of mechanics wire while you work on it,

Caution the alternator harness has a a thick red wire and one thin blue wire running in the silicone sheathing ,the thin wire can shear off just outside the black cover if you let the alternator hang from its harness so use mechanics wire to hold it with slack in the harness.
After removing the rear cover remove the regulator check the brushes for wear if they are short then either replace the brushes or replace the regulator only use Bosch parts for a new regulator as the off brands will not provide the best service life .
Most alternator issues are due to failed brushes or a sheared blue wire, (the blue wire runs through the 14 pin connector above the hot post)
Refit the alternator back cover being careful with the harness so its not pinched, then fit it to the engine, make sure the air duct tube is also refitted as this tube has a temp sensor in it , the alternator fan pulls air past to let the sensor compare outside air to inside air.

NOTE most of the elex issues should be corrected if you have done all of the previous instructions.

The warning light boxes are under the dead pedal one black and one yellow connector they dont usually cause problems.

If your still having issues then

Next is to remove the gauges/ pod and clean the slip on connectors there are 3 of them use the eraser so the foil tins are clean ,
please report back prior to pod removal for further instructions