I agree, in terms of the entire vehicle assembly however, my point being that less complicated assemblies such as white goods and some electronics for home application have an acceptance of such a high level of defects in comparison to what is expected and achievable within the automotive component manufacturing industry. I know that quality control and quality assurance do come with some additional costs, especially setup costs. One of the problems with a vehicle assembly is that with so many components, there only needs to be a couple sub-standard suppliers and it will increase the risk of a higher failure rate.
With white goods and electronics, I am still amazed to see this acceptance level make it out to the consumer although I do concede that an AVR or projector does have many things that can go wrong and often are at the mercy of their component suppliers at that price point.
I just realised how old this thread is. BriziAVGuy certainly pulled up an old thread here.
When combining signals use as near as possible identical signal levels and the combined total will rise by a maximum of just under 8 dB, however considering that the signals are very different to each other the rise will be much less. You must prevent intermodulation distortion in the following amplifier. You must ensure that the output level from that amplifier is less than its maximum output level by at least 10 dB to prevent unreliable decoding. Measure the Pre-BER as the number of errors rises as intermodulation distortion increases. This will be affected by the amplifier gain setting and any attenuator you insert between the combiner and the amplifier input. To get the optimum you should measure the Pre-BER.
The RF channel selection will also affect the level of errors. Remember that intermodulation distortion produces frequency1 + frequency 2, as well as frequency 1 - frequency 2. At lower powers multiples of these frequencies also occur.
On a single part ppm is realistic, but keep in mind that a product, especially a car, is made up of hundreds/thousands of parts, so the combined product fail rate is likely to be somewhere between 0.1 - 0.01 (one in a thousand to one in ten thousand) - in a well managed quality control setup.