I did some looking tonight and figured out how to rewire the fog lights are on the '00 S4/A4. This mod probably applies to the '99.5 A4 too. Doing this mod allows the front and rear fog lights to operate anytime the ignition is on. The headlights or running lights don’t need to be on and the fog lights won’t cut out if the high beams are on. This mod does NOT allow the rear fog lights to come on independently of the front fogs though.

This mod requires NO cutting or modification of the wiring harness. It does require the modification of the unit that controls the fog light buttons. But, this part can be easily unplugged and replaced if you have any problems. I don't think it's very expensive either because it is a fairly simple device.

To do this mod you will need to get the glove compartment out of the way. The best way to do this is to open the glove compartment and locate the stop tabs that hold the glove compartment in its normally open position. They are located at the top rear corners of the glove compartment. Once you find them, squeeze the sides of the glove compartment near the stops. Doing this should allow you to open the glove compartment past the stops.

Once you have the glove compartment out of the way, look inside, up above the upper right hand corner of the glove compartment opening. If you feel around you will find a rectangular relay-like module. It’s about 2”x2”x1”. Once you find it, you should be able to basically pull it out; it wasn’t that hard to get out. You’ll then want to pull the module out of its socket.

You need to cut off pins 12, 15, and 17 at their bases on the module. Next you need to internally jump pins 15 and 17 to pin 14. To do that, open the module like a normal relay. Inside is a small PC board. Notice where all of the external connector pins attach to the PC board. You need to solder the connectors that correspond to pins 14, 15, and 17 together. Fortunately they are all right in a row and easily traced. Pin 17 is the second position in from the edge of the PC board. Position 3 is unused, 4 is pin 15, and 5 is pin 14. Solder the pins together and you are done. Just put it all back together.

Some notes:

For those of you who want to know more, here is what all of the pins on the module appear to be:

Pin 1 - +12V pulse toggles front fogs
Pin 2 - +12V line to rear fog
Pin 3 - +12V pulse toggles read fog
Pin 5 - +12V line to front fogs
Pin 6 – Appears to be ground
Pin 7 – Appears to be ground
Pin 8 - +12V switched with the ignition
Pin 9 - +12V switched with the ignition
Pin 10 – Panel indicator light for front fogs
Pin 11 – Panel indicator light for rear fog
Pin 12 – Holding at +12V PREVENTS the front fogs from turning on (used to cut out front fogs when high beams are on)
Pin 14 - +12V switched with the ignition
Pin 15 – Holding at +12V ALLOWS the rear fog to turn on (used to cut out rear fog when high beams are on)
Pin 17 – Holding at +12V ALLOWS front / rear fogs to turn on (used to prevent fogs from turning out with out the low beams)

If you want to be able to turn on the rear fog independent of the front fogs, you’ll have to modify the control module. Personally I want to maintain this feature. Originally I wanted to restrict the fog lights from coming on without the running lights. But, there appears to be no way to do this without rewiring the wiring harness. This was out of the question for me.

Disclaimer: (As James Wu said in his fog light FAQ) North American law requires that any accessory lighting (i.e. fog lights) on a vehicle be only operational when the vehicle's low beams are on. The factory fog light wiring for the Audi A4/S4 sold in North America conforms to this law. By performing the following modification, you are bypassing the factory (lawful) wiring, which prevents the use of the fog lights except when only the low beams are on. Please note; that it is now up to you to obey this lighting law, and only use any non-standard lighting patterns now available when off of public roads. Also note that the electrical system on any modern automobile is very complicated. Please be sure you know what you're doing before performing any modifications to the electrical system. This rewiring has proven to work on the author's own vehicle. However, please note that the author will not be responsible for any tickets, fines, damages, or personal injuries; which occur before, during or after this modification.

Plainly put if you can’t solder, harassed by cops at the local doughnut shop, or lynched by the anti-fog light posse on this board; I’m NOT responsible! :o) Good luck!

Peter

'00 S4 Santorin/Silver/Tip