On Robin's scale of difficulty (1 to 10) I would rate this task a 2.
Much of the information is derived from Randall's
investigation on the oil cooler fan. You should review this information first:
964/993 Oil Cooler Fan Operation & Troubleshooting
The difficulty really depends on where you decide to mount the switch. There have been several common options:
Almost any switch will work, provided that it is NOT a momentary switch. Be aware that for normal
fan operation, the connection will be shorted. This only matters if you decide to use the rear
fog lamp switch becuase it will be pulled "OUT" for normal fan speed operation, and pushed
"IN" for high speed operation.
For the simpliest install, it takes about 1 hour for the wiring, 30 minutes to modify the switch.
I'll cover the common parts of the installation first, then talk about specifics of mounting the switch in the console, dash, and the rear fog lamp locations. If you are planning to do the dash install, read the information about mounting the switch to the dash first!
In the driver's side footwell, you'll see two bunches of wire going up to the CCU. The CCU is the unit to the left of the stereo ( when sitting in the driver's seat). You'll see an extra "bend" in the two cable bunches held by electrical tape. This extra bend is just used to take up the extra slack of the two cable bunches. Cut this piece of electrical tape to allow more slack in the two cable bunches.
Insert the DIN removal tools into the slots on the side of the CCU. Push them in so you feel/hear a click. Then pull out the CCU. You may need to push the CCU out from the back, becuase the DIN removal tools do not always catch postively enough to pull out the CCU from the front. You can use the big end of a screwdriver if your fingers can't reach.
You can also use 4 small nails or two pieces of coat hanger wire. Either way, DO NOT PULL THE CCU OUT USING THE KNOBS. You run the risk of damaging your CCU. CCU's cost about $1000 to replace.
Pull the CCU part way out, you may need to move the steering wheel stalks a bit to get clearence.
You will see two cable connectors on the back. These connectors have a spring loaded clasp on one side. You will need to depress the spring loaded clasp and then gently work out the connector. The side of the connector with the spring loaded clasp needs to come out first! The connector will not come "straight" out. When you get one connector out, you'll understand what I mean.
On the larger connector, look for the G12 wire. It's the green/black wire. That is the wire that you want to tap into. Verify that this is the correct wire several times by looking at the connector. The wires are numbered on the connector. You don't want to cut a wire that you don't have to.
After you are sure that this is the correct wire, then cut the wire. On each end of the newly cut wire, you are going to attach your speaker wire. You can either use wire taps or solder your connections.
If you are using the dash switch, I would recommend cutting cutting/tapping into the wire about 12" from the connector to the CCU. If you are using the console or rear fog light switch, I would recommend making cutting/tapping into the wire about 4" from the connector to the CCU.
Reinstall the CCU, and attach the speaker wires to your switch ( see below )
With the key in the ignition and switched to on ( You don't need to actually start the car ), turn your switch on and wait about a minute. You should hear the high speed fan come on. If this doesn't happen, try setting the switch the other way, and wait another minute. You may have the switch upside down or something like that. Try it both ways, becuase your switch will turn the high speed fan both ON and OFF.
If the high speed fan does not come on still, then you'll need to verify that your high speed fan is operational, your CCU is operational, and double check that you have shorted the correct wire on the back of the CCU.
Assuming that your switch works properly, use the nylon ties, electrical tape,etc to clean up your installation and make sure that you do not have any loose wires in the driver footwell.
Additional Notes on Switch Locations
When mounting a dash switch, you will need to remove your dash first. To remove your dash:
Console switch mounting
The center console switches are mounted on two plastic pieces that can be pulled out by hand.
Pull out the bottom bank of switches first, then the top bank.
You will need to modify the switch slightly with a dremel tool to fit properly.
Rear fog light mounting
I haven't done this before, but when I did my oil cooler fan switch install, I wired in the back light to my fog light switch at the same time, so I was working in the same area. I did not use the rear fog light switch becuase of the "backwards" operation of the switched mentioned earlier. However, that was just a personal perference.
You will need to remove the stereo to do this install and find an available grommet to push your wires to the switch. Use a continuity meter to determine which pins on the back of the rear fog light switch to use.
If you haven't installed your rear fog light switch yet, I'd recommend that you do this first. It takes quite a bit of cutting/shaving of the dash area to get the switch to fit right. You can make a tool out of a coat hanger to screw in the small circular faceplate of the switch. The correct size for the hole which you need to drill in the dash is 3/4".