IC means integrated circuit.
In the typical HU, when you need to connect two audios ICs together, you need a coupling capacitor to block the DC biasing voltages from one IC from bothering the other. The main reason for this is due to the fact that you usually only have a single supply voltage like 5-12 volts to power the IC. The ICs audio pins will have 1/2 the power voltage on them. The same goes for when you want to feed an audio signal from an IC to the outside world. There will be say 2.5-6 volts DC offset on the input/output pins of the IC. The audio signal "rides" on top of this DC offset voltage unless you block it with a capacitor. In your case the amp you are sending the signal to probably has input coupling caps blocking this DC voltage. Be careful not to drop your RCA connectors on a ground point. You might pop your HU.
Some designs use very few coupling capacitors, but the ICs are specifically designed to use the DC offset on the output of one IC to bias the input of the attached IC. Other designs use very tiny ceramic chip capacitors for coupling. GM HUs do this everywhere. You can only use a small value coupling capacitor if the input impedance of the following stage is very high.
As far as measuring AC, if you use a cheapy voltmeter set to the AC scale, it blocks DC when measuring AC. If you use a True RMS voltmeter, it will measure the DC offset as well as the AC signal. You will see a "signal" voltage even with the volume muted.
Another BIG thing to mention is that you have exposed an internal IC to a world of hurt by not putting some form of transient protection on each out put. As a minimum I would add two diodes and a 100 ohm resistor plus the blocking cap.
The schematic would be like below:
IC +
_|_
/\ 1N4002
+ |
IC Output ---| |-------+---\/\/\/\/-----o Pre-Amp Out
10 UF | 100 ohm
_|_
/\ 1N4002
|
IC Gnd