Help needed on audio amp setup

This is probably going to be very long winded and keep in mind that I'm inexperienced when it comes to car audio.I installed a Toyota navigation system from a 2004 Toyota Solara into a 2004 Toyota Sienna. The system is just about plug and play with the exception of the voice navigation signal. The Nav unit put out what appears to be a preamp'd voice signal. I was able to take this 2 wire out signal and input it into a radioshack 7.5watt amplifier and drive it into a small 3 inch speaker. Problem was that the RS amp is just not enough. At max volume out on the Nav system the voice is hard to hear at highway speeds. I then tried to install an older JBL (18w @ 4ohm) amp and here is where I'm having problems.When I input the voice out into a single channel of the amp through the low level input the sound was more than loud enough at the mid range of the volume control of the Nav unit. I had the line level adjust on the amp at minimum and it worked until I turned the ignition to ON or started the engine. As soon as the key was rotated to ON the speaker would emit a buzzing/noise that did not increase or decrease with the NAV volume control but was pretty constant. If I returned the key to the ACC position the speaker buzzing/noise would go away.I then connected the voice out signal to the High level input on the JBL amp and tried again. When set up this way the sound was clear and the buzzing/noise was not there in ON or when the car was running. Problem was the sound is not loud enough even a the highest setting of the Navigation volume control in this configuration.I realize that the low level signal probably amplifies that signal greater then the high level signal and in turn probably amplifies the noise also. But when I used the high level input there was no noise, not less noise but no noise.Some details that might assist helping me:I was using the cig lighter/accesor plug for powering the amp as a temporary solution so the ground for the Nav and the cig lighter/accesor plug were not shared. I could try making sure the ground is shared and try it again but I'm not holding out any hope.Any assistance would be appreciated.Thanx,jeff