I just finished my audio install in the 65 mustang convertible, and it worked just fine.
I wanted to replace the aftermarket am/fm/cassette radio with an original am radio and install kick panel speakers, but needed to also be able to plug in my ipod, which I had been doing via that cassette adapter.
So, I installed a Pioneer 4 channel amp in the trunk, ran the cables to the front. under the scuff plate. I bought the kick panels from Canadian Mustang in BC, bought the Pioneer low profile speakers on Ebay cheap and installed them. Also got a subwoofer for the trunk (more on that later...)
I have an RCA cable running from the Amp in the trunk to the dash, tucked inside the console storage.
For the Ipod, I have a 1/8" phono to RCA cable that connects into the Amp's RCA cable. For the recently acquired original am radio, I used a line converter to convert the speaker output to RCA.
Since you can't have two inputs connected to the RCA inputs on the Amp at the same time, I simply manually switch the cable, hidden in the console storage. I will eventually get an RCA switch.
What an improvement in sound over the 4x10 speaker. AM radio is still mono, of course, but sounds better, and is louder. The ability to use my ipod is great, and running it thru an amp and decent speakers makes the experience that much better.
As for the subwoofer, it doesn't do much. I think it's because it's a convertible, so the base doesn't resonate between the trunk and passenger compartment, it sort of 'leaks' out the top where the ragtop is stored. May end up pulling it out, but will leave it for now.
Michel.