My dealer would not install a radio and believed that it might interfere with the car's computer.
(1) Does anyone have any experience with late model Devilles and VHF? Any issues?
(2) Does anyone know if there is power available in the trunk or would I need to run a power cord back to the battery. I understand that going directly to the battery is the preferred method, but if it works well, good. If not, I can always run a power cable.

Does anyone know of a 2/440 dual band fm rig for vehicle with a mike display of frequency/channel ? Yhis would really simplify installation.

The relationship between bandwidth and power, for fixed S/N, is fairly straightforward and is easy to derive from Shannon's work. That relationship is:

PWR ratio in dB = 10*log(B2/B1)
---where log is log base 10, B2, is the bandwidth of channel 2 [ssb ~2500] and B1 is the bandwidth of channel 1 [say, psk31~31.25].

3.125 Hz is interesting as many waterfalls I see show distinct 3rd IMD and even higher. I am assuming the transmitter is properly adjusted and linear.

To get the power invert the dB back to Power:

P2 = P1 * 10^(dB/10)

---where P2 will be the equivalent power to achieve the same S/N on channel 2 as P1 achieved on channel 1. dB is the dB you calculated in the first formula.

You do get figures similar to what are discussed in this email thread. In fairness to ssb, there is the potential for a lot more information in its bandwidth and there is also a relationship between signal levels and required bandwidth. It is these relationships and their consequences that were the core of Shannon's work.

It is important to remember that what Shannon was driving at was channel capacity in relationship to noise and your results may vary based upon band conditions. He assumed white [random] noise and that 1.5 KW station next door 3 Hz away produces different issues.

Hope this helps.

Rob WW6G