An alternative location is low in the door or side panel, just behind the front seats. The front seats block the direct path from speakers to front seat listeners. If the rear speakers cannot be placed optimally, a high-end roll-off will keep them from pulling images to the rear, but will still allow them to give a sense of space. Assuming the system is designed to sound best from the front seats, frequency response of the rear speakers is not too important as long as it adds in a pleasant way to the front speakers.
When playing the system, set the fade control just far enough forward to ensure that no images leave the front stage and fly to the rear. This keeps the musicians on stage and out of the trunk. This basically conventional system optimized for ambience reproduction will sound more like a living room than a car, and thus more like a live concert.
Enhancement Devices
One popular approach to getting better ambience is to synthesize it. A reverberation device with adjustments allows the user to match the original acoustic space. A heavy dose of reverb is then played out of the rear speakers. The usual problem is that the resulting reverb is just too much. If we could take away the recording's or the car's acoustics, it would work better. This method is not ambience extraction or reproduction, but rather, ambience synthesis. I reserve ambience synthesis for generating spectacular effects. It can be a lot of fun.
Non-reverb ambience-extraction devices can be identified by suddenly switching off the input while playing music. If the output instantly switches off as well, it may be an effective ambience extractor. If there's a dying echo, sproing or boing, the device is in the synthesizer class. The extractors usually subtract the right channel from the left channel to generate the L-R signal. Any sounds recorded equally in each of the original stereo channels are cancelled in the L-R signal.
Sounds cancelled include center stage instruments and voices and most low bass. What's left is echo and things that come from the sides. The L-R then goes to the rear speakers directly or through a short audio delay. Delay ensures that pure left and right front-speaker sound arrives first, establishing a front soundstage direction. The L-R arriving from the rear can indeed bring out recorded ambience and lend a nice spacious quality.
Dolby surround sound or film has influenced some ambience extractors. The true Dolby technique decodes a specially encoded stereo program to control image locations by "gain-steering" of its output signals. Related ambience extractors use ordinary non-encoded stereo to control gain-steering circuits. Special circuits combine left and right inputs to produce new left and right outputs for the front and rear, as well as a mono or L+R signal for a center speaker. All outputs are usually gain-steered to produce spectacular imaging and ambience effects.
Thus, a strong center soloist may cause increased L=R gain, decreased left gain and right gain and slightly increased rear gain. In this case, the soloist would be strongly centered on the stage with increased ambience. However, some kinds of music can trick the gain-control timing. This leads to surging images or collapsing ambience. Nevertheless, I've heard a couple of very good systems of this type.
Recreating the Living Room
I claimed earlier that the basic problem of ambience reproduction in a car is that its acoustics are not those of a living room. This suggests a solution; modify the car's acoustics to make them more like those of a living room. This requires reducing early-reflected sounds (those that arrive within the first 5 milliseconds after the direct sound) and adding side arrivals after about 10 milliseconds.