This can indeed be confusing, as I have to deal with inquiries about this every day.
Here's what your original car has:
An antenna connector -- this should be easy enough, most all radios made since 1940 use the same motorola connector, just plug it into the antenna jack.
Power connector -- on all 64-66 Thunderbirds this is a two-pin connector, with a standard-sized bullet pin (black wire, or yellow wire with black strips) and a smaller pin (always a blue wire with red stripe). The larger pin is power to the radio; the smaller pin is power to the lamp.
Speaker connector -- this is a two-pin connector with one male, one female bullet pin. If your car has a rear speaker option, this is a four-pin connector. The reverse pin is ground. The pin next to it is speaker output. The other two pins vary a bit, but are used through the fader control. On 64's one os front speaker return and ones is rear (can't remember which is which). On some 65-66 models I think one is a front return and one is a midpoint or somesuch. Swapping radios between model years can be an issue with these, as the fader control might not work properly.
Now -- you're replacing the original radio with a USA-230. You can connect the ignition wire to the big pin of the power connector. If you have a separate wire for dial lamp (some of these do and some don't), connect it to the little pin. The ground wire needs to connect to a metal part of the car -- fasten it underneath a screw or whatever. The battery wire is used for memory, and must be on at all times, so find a place in the fuse block that has battery power when the ignition key is off.
For speakers -- yes, you DO want new speakers, and I suggest Custom Autosound Dual Voice Coil -- DVC 3001 for front, DVC-3015 for rear. Someday I'd like to try fitting a 6x9 in the back (DVC-3006), but the 3015 is the one that fits between the studs. Also, it's wise to fill in the gap as the speaker sits back from the package tray -- use foam weatherstripping. DON'T use the original speaker wiring -- wires are connected together and one of them is grounded underneath the glove compartment. You need two wires to each speaker, or four wires if they are DVC -- each speaker is electrically two speakers. Suggestion: if using two DVC's, connect both left channels to the front, both right channels to the back. You'll have full stereo effect and full power. Set fader in the center, adjust balance between front and back.
One more thing to check -- there is a white connector that plugs into the back of the radio. Custom Autosound has improved their quality dramatically over the past few years, but this connector is still a weak point (the new slidebars no longer have these). Make sure all the pins have a solid connection, to avoid intermittents.
Installed properly, you've got a stereo you can really enjoy.
