Q: I have a factory CD player in my '98 Nissan Altima and am going to install an amp. How would I hook up a remote wire to it? Is there one already in it? And, if there is, how can I figure out what color it is? If I wanted to add two amps, what would I do different? Would I use one remote wire for both of them?
A: I'd always first default to using an aftermarket OEM interface. Throughout this issue of CA&E there should be many examples of manufacturers, so the choice is yours. This is an important point relating to one of the many reasons that stock sucks. Like so many other examples of factory gear designed right to the limits of the product's capacity, very few OEM source units have the ability to drive more than one anemic factory amplifier. Try connecting an aftermarket amplifier, or more than one of any brand name amplifier and the remote turn-on lead from the factory source unit is likely to fry.
The output capacity of the remote turn-on lead is limited to only what the carmaker would offer for gear, rather than being ample to allow for an upgrade. I imagine their philosophy is: "My cassette tape deck sounds great -- why would you want to upgrade this?"
To find out if your radio has a remote turn-on lead, use a digital multimeter set to the DC voltage setting, and if the meter has a scale switch, select the next number over 18 volts. Ground the black probe to the vehicle's chassis (never a factory ground wire) and test each wire coming out of the back of the factory source unit. As you test each wire, turn the radio power switch on and off with the volume at minimum. You should see the voltage jump to over 12 volts with the switch on, and back to near zero with the switch off.
Once you find this wire, leave the radio switch on and stick in a CD (or press the source button if there is already a CD in the transport). The voltage should stay at pretty much the same level. If the voltage plummets to zero, you may be on a power antenna trigger lead, which isn't usually suitable. On some radios the power antenna trigger and the amplifier's remote turn-on lead are the same thing, but on other radios, the power antenna would retract when you were playing a CD, since it wasn't needed at that time. Only the amplifier turn-on lead would remain live as long as something is playing through the source unit.
Next, test the capability of the wire. For this you'd need a few 0.5-watt resistors and a calculator. Start with values around 330 ohms and connect a single resistor between the wire you identified and ground, while you keep the red probe on the wire. Your reading will drop very slightly due to the 330-ohm load resistor you added, which Ohm's Law calculations tell me is almost 40 milliamps (0.04A) of current draw. (We determine this by dividing 12.6 volts by 330 ohms, equaling 0.038 amps). The fact that the voltage stayed up means that the output can deliver at least that level of current.
Now connect a second 330-ohm resistor in parallel with the first one and monitor the voltage. It should still stay roughly the same, even though double the current is being drawn. The resistors might get warm, but they're still dissipating less than 0.5-watt of heat per resistor. Keep adding resistors until the voltage either drops a big amount quickly or is below 11.5 volts. This is the point where the limits of the radio are revealed. For example, if you had five resistors in parallel and the voltage suddenly sagged a bit below 11.5 volts, we have confirmed that the amplifiers' remote turn-on leads can draw no more than 190 milliamps (0.19 amps). This was calculated by taking five resistors at 330 ohms each (330 divided by five), which is 66 ohms in parallel. 12.6 volts divided by 66 ohms equals 190 mA.