[Android Head-Units] kaween:
Ah, the Zhoon guys&girls. Friendly people, but not hampered by a thorough knowledge of English and you (well, I) can only get the correct info from them after asking the same question 4 times over. :D

Anyhow, basically he's right, but as always, he/she's not complete while providing the info. Let me give it a try.

The AN21 has 2 cables coming out, named KEY 1 and KEY 2. It ACTUALLY has a third wire too acting as "MASS", and that is your black GROUND cable which must be common througout your car. Better said, if you'd measure from your "-" cable of your battery to ANY other metal part of your car, INCLUDING your radio, the resistive value -in ideal cases- would be ZERO.

Anyhow.

Bascially,what the unit does is it puts a voltage out on these pins (roughly 3.85 Volt), and the idea is that when you press a key on your steering wheel, it translates in a resistor value, Older cars MAY or MAY NOT translate a keypress to a resistive value. MOST modern cars will NOT. In that case, some basic electronics provides a translation from "a pressed key" to "a certain resistive value".

The radio measures this value, and you can "hook" a certain function or action to that switch. Hence the term "resistive learning".

It's actually quite a nifty system : low cost, very flexible, very cheap to implement.

To correctly measure a resistance, the unit needs a "0-line", a "zero" so to speak. That line is your "Mass". You know this line, but you will call it "GROUND" (it's the same thing). So the device that measures a resistance MUST have a connection to the "resistor" it wants to measure (call this measuring line "key 1", or "key 2", and will compare that value to the "MASS" or "Ground" if you will.

Hence, your radio's key 1 OR key 2 (depending on system, you may well get away with using just ONE of them and leave the second one unused !) must be connected to ONE end of the SWC or SWC-interface, and the OTHER must be connected to the SAME GROUND as your radio is having its "ground" from (and yeah, that should be the SAME ground throughout your car, basically it's the metal chassis of your car which is connected to the "-" terminal of your battery).

If your specific SWC-interface provider says it has 3 cables as output, 2 of them will be "signals" connected to Key 1 and Key 2. The Third will be a MASS, which you will have to connect to the SAME GROUND as the RADIO is connected to. So you can basically "create your own third cable" for the SWC by simply connecting the GROUND of your radio unit to the MASS of the SWC interface.

Mind you, I have VERY bad experiences with most SWC control boxes I've come across, So I've build one my own for my cars. :)

If you connect KEY 1 and KEY 2 and MASS, and the radio would not "see" anything, try using only ONE KEY cable and leave the second one unused. If that doesn't do the trick, connect the other KEY cable and leave the previous unused. Whatever you do, you will always need "MASS/Ground" for the thing to work.

If your SWC-interface would only have 2 cables, you'll PROBABLY need to connect ONE of the "KEY" wires to one of the SWC interface, and the OTHER cable from the SWC will have to be connected to that same ground level as your radio is. In case of doubt, just post what kind of interface you will be buying and we'll have a look.