Permission to publish this article is provided generously by Robin Sun at www.p-car.com. Be sure to visit his site for loads of 993 info!
When I moved into my house a few years ago (after having shelled my money out to a landlord for a townhouse rental for umpteen stupid years) I quickly got accustomed to the installed garage door openers. Leaving the house and coming home and not having to open/close a garage door by climbing in and out of my car was a nice convenience in miserable weather. However, I figured there has to be a better method than the hand-held transmitters that came with the house. Having noticed that a popular new option in cars nowadays is a built-in garage door opener, I decided to try to install such in both my '95 993 and '98 Audi A4. The method I took was a bit different on both cars, but I figured I'd pass them on to y'uns for your use (and amusement).
First, tools. You will be doing some wire installation in your pride and joy. To me that means soldering any wire connection and protecting it with shrink-wrap tubing (the days of using crimp-on connectors or tap-ins and covering any connections with electrical tape have long disappeared for me). I have gotten to like these little butane-powered soldering irons. In any case a low-powered one of 50 watts or so should be fine; these big soldering guns that look like a power drill are likely overkill. A volt-ohmmeter will be handy to find potential voltage sources. You might also be doing some desoldering on a printed circuit board, so a desoldering tool could be handy. Plus a nice trouble light for your in-car work (pref. fluorescent to avoid burning yourself, the carpeting, plastic trim, etc.).
Now, I took two different routes with the Porsche and Audi. With the Porsche, I adapted a hand-held unit. The Audi already had a built-in unit as an option on later cars, so I bought one of these and hard-wired it in. These are some of the details on each installation.
A) Porsche Aftermarket
This installation was basically taking one of my existing hand-held transmitters and hard-wiring it into the 993. I had a spare spot on the console for another switch, so I bought another switch from Porsche and installed it in this spare location. For a garage door opener, what you need is a spring-loaded, momentary contact switch. I found the switch for my Check-Engine warning light ideal, so I ordered another one and used it (see Figure 1; the new switch is at the lower left). You can either do something similar on your car (I think most cars have some type of spare spot on the switch panel), or else just buy a momentary contact pushbutton switch and mount it in a convenient spot under the dash.
If you are adapting a hand-held transmitter unit, there are two things to consider:
a) power: the battery it uses should be a 12V one. If so, then you can hard-wire the unit to a power supply from your car. I had already tapped into my fuse panel a few years ago to install power leads for my radar detector and CB radio, so these power leads were already routed into the car interior. If you have a car model from the '90's or newer, you likely have blade-type fuses. Then tapping into a power source is easy; see Figure 2 for my Audi connection. This is a "fuse tap" you remove your existing fuse and plug this unit in. Two fuses plug into this unit; one is the fuse you yanked to install the tap that completes the car-original connection, the other is a separate fuse for the new accessory that needs 12 volts.
b) Memory: in a sense this goes along with power. The concern is will the unit keep its operating code with no power supplied to it. One way to test this is to take the battery out of your hand-held unit and replace it, then see if the transmitter still operates the garage door. If it does, you can use either a switched or unswitched power source ("switched" source means you lose power when turning the car off; "unswitched" source means power exists anytime). If you lose memory, then you need to ensure you tap into an unswitched power source. You can use a voltmeter to check for power to the various circuits; look on top of a fuse and you'll see an opening on either side of the fuse. Leave the fuses in place, and check for switched/unswitched circuits by connecting one voltmeter lead to this fuse opening and the other to ground.
Having located a power source and a likely actuation switch, you now will to have to wire both into the transmitter. Figure 3 shows my installation on the hand-held transmitter I wired into the Porsche (on the left), along with an unmodified transmitter (on the right). I desoldered and removed the battery contacts from the circuit board and ran the power leads to the circuit board contacts (WARNING ' take careful note of power lead polarity!) and soldered them in place. For the pushbutton switch leads, I did some continuity checks on the switch on the circuit board that is normally actuated by the transmitter pushbutton, and decided to wire onto this switch. This needed desoldering the leads, connecting the new leads to the switch contacts, then resoldering the leads to the circuit board (the switch had two sets of contacts connected to it, and I didn't feel like experimenting with removing the circuit board switch entirely). One other thing; you'll notice at the right center of the unmodified transmitter (by the designator "D") there is a component which is removed from the in-car unit. This is an LED that lights whenever the hand pushbutton is pressed. I had read that additional transmission power can be gained by removing this LED (all power now goes to the transmitter). Whether it is true or not I don't know, but I figured I no longer needed the LED so I removed it.
B) Audi Factory Unit
I noticed that more recent Audi A4 models have a "Homelink" garage door opener as an option. This is built into the driver's side sun visor. Figuring that an installation would be easy, I bought a visor/opener unit off of E-Bay. However, they had changed the visor design and coloring on the later cars, so this visor wouldn't match. Also, I would have had to try running a power lead up through the A-pillar or across the roof interior, and this was a bit more than I wished to do. Instead, I installed it in the overhead center console by tapping into some power leads for the overhead map lights. So, Figure 4 is this transmitter mounted onto an overhead removable trim panel. One nice thing about the "Homelink" transmitter--it maintains its memory even on power losses, so you can wire it up a switched 12V power source. It is relatively compact also, being about 3" by 4" and needs about 3/16" of depth clearance. You can also set it up to open as many as 3 different garage doors.
One other thing to note: locate the transmitter in as clear a non-metal surrounding area as you can. I went nuts installing the Porsche transmitter because I was trying to put it in the center console behind the switch unit. This area is constructed of fairly heavy steel, and it acted like a complete signal barrier! I lost count of the number of times I pulled out the transmitter, held it out in the clear to find it could open the door from the bottom of the driveway, but when I reinstalled it the transmitter wouldn't open the door even when the car was directly under the door operating mechanism as it sat in the garage! Success was only obtained by installing the transmitter in the rear section of the console whose cover is plastic and vinyl.
Well, that's my installation. Got comments or other helpful hints, pass them on.