Having good music, a radar detector, and an intercom have made my riding a lot more relaxing, especially on long trips. Of course, my bike didn't come outfitted this way and I learned a lot in the process of assembling my electronics. Hopefully if you want to go this route you won't need to reinvent the wheel regarding some of the basics that I'll discuss here. A lot of this will be brand-specific; that's unavoidable since the installation techniques vary depending on the equipment. Still, the principles are common to all electronic work.
Before you do anything, establish the following.
My initial efforts were severely compromised by poor materials. The result was poor reliability and at least one important fuse blown while riding.
The K-bikes are very bad in this regard. My bike had a very load whine from the fuel pump and lesser noise from the ignition, computer, and injectors. It is VERY important to have good grounds for the whole system and to shield the cables whenever possible. Keeping the electronics in the back of the bike helps keep their cables from picking up RF noise from the computer. Run a separate (and heavy) power cable from the battery to power only the sensitive electronics.
My basic rig consists of an Autocom (intercom) with a Valentine One (radar detector) and a Cobra CB. There's a "Black Box" for connections and supporting circuitry.
I started with a box from a Braun electric shaver (it was a good size) and an old cigarette lighter power adapter from Radio Shack. I needed to get 9V for the Autocom, so I took the circuit board out of the adapter and unsoldered the various wires from it. Trimmed a piece of perforated circuit board to fit and mounted the components on it. Holes in the box with rubber grommets passed wires through. Also in the box is the V1 audio adapter.
The input points on the circuit board are used for junctions wherever possible. Essentially the power goes from the bike into the box, through an inline fuse, and to the 12V input terminal of the power supply. Also soldered to that point is the positive power lead of the RJ11 cord to the V1 audio adapter. The negative lead goes to the ground on the circuit board and the ground RJ11 lead connects here, too. Power from the circuit board output goes to the Autocom and is delivered through a 9V pigtail (same as comes out of the Autocom) and these pigtails are available at Radio Shack, too. Remember that the polarity may appear wrong on the pigtail! I removed the flimsy wires on the 9V pigtail and soldered on heavier ones.
The result of this black box is that I have power to the Autocom and the V1, and the audio adapter is protected inside the box.
The V1 is a very rugged radar detector. All of the accessories for it (remote display, remote audio, power supply, etc.) are interconnected with standard phone cable, so you don't need to go to a proprietary source for replacement cables.
The power to the unit is fed from the two center wires out of the four in a typical RJ11 ribbon; I used a Radio Shack RJ11 pigtail (with lugs on the ends of the wires) and the red was negative and the green was positive. Don't try to cut the end off a regular cord, since it is likely to have foil conductors which are impossible to reliably connect to anything. The RS pigtail has real wire conductors. That part goes from the input power to the audio adapter, and a separate RJ11 cable goes from that to the V1. Due to corrosion, the cable to the V1 needs to be replaced periodically. The terminal to the outside of the ground pin carries the audio, and can be taken directly to the autocom if you want (no volume control on this signal). The first pin, next to the power input pin, carries a digital signal.
A disadvantage of this setup is that I cannot adjust the volume on the road (since the audio adapter is inside the black box) but in practice no adjustment is needed. The V1 usually rides in the map pocket of the tankbag.
The signal from the headphone output jack is stereo, but the two channels are bridged with resistors. You should use a stereo plug so that it is properly held in the jack. If this is connected to the stereo jack on the Autocom, you can use a prefabricated cable with a stereo plug on both ends. If you are connecting to the transceiver plug, you'll need to join the two channels together. In this case a mono plug might work at the V1 end.
Don't try to use the speaker jack on the audio adapter for this purpose! It is a mono output with a floating signal and gets extremely unhappy when either side is taken to ground. Since all components in this system have a common ground, the speaker jack cannot be used.
The Autocom Pro 3000 takes 9V power, has a stereo input jack and cables with male 5-pin 45° DIN plugs for the rider and passenger headsets. There is also a female 5-pin 60° DIN plug (the transceiver plug) for interfacing to a 2-way radio, cell phone, or any on ther input/output source. Using this plug can be straightforward or difficult; see the separate Autocom page for more details..
I have located various places, like in the tail trunk and in the tool tray, but generally I put stuff in the tankbag now. There's a lot less dust and vibration, and I have easy access to controls.
I have used several different CBs with this setup. See Autocom and radio integration for more details.
Hopefully your devices take 12V power straight from the bike, but if not, there are two main options for getting power without resorting to batteries:
Note: some devices may not respond well to a third-party power supply! If you decide to use one that's not provided by the manufacturer of the device, you could damage the device.
Buy a 12V-to-(x)V converter. These can usually be found at Radio Shack in the right configuration. Also, if your device was intended for use in a car you may be able to buy the correct 12V adapter from the manufacturer. I have in the past taken cigarette-lighter type converters and removed the internals, then wired that circuitry to the component (Autocom, radio, whatever) although simply providing a bike-powered cigarette plug and then plugging the adapter into that is the same, just bulkier. These days, the whole device is usually barely larger than the plug itself.
Make a power supply of the right voltage using a power supply IC or an LM317 IC. Total cost is only a few dollars. If there is an appropriate IC already made, it's as simple as connecting input voltage, output voltage and ground to the three pins, along with filtering capacitors across the input and output. Three parts and a few solder connections. If the right IC isn't available, an LM317 can be used with only a bit more effort. One pin goes to input voltage, one to output voltage, and the third takes a partial voltage from the output for control. Either of the "circuits" pages below will give good examples.
PowerWerx (distributes Anderson connectors)
Posi-Lock connectors (reusable, no-solder)
Bill Bowden's Hobby Circuits (more good stuff)
Powerlet Products Electrical connectors