What is your original radio in the car?
You'll find one of two things, either the early style one-wire speakers used only with the Motorola shaft-mount radios, or the more normal two-wire setup common with later radios and the DIN radios using one of two kinds of connectors. Being on the edge of the transition, your 85 may have any of the wiring setups.
The front speakers are normal 5-1/4", and the rear sedan are as well. I'm not sure what the Wagon used, probably the same thing.
I believe you can order the adapter for the later system. The pinouts are here:
http://www.amceaglenest.com/~iowaeagl/guide/index.php?title=Radio_Pin_Outs_-_AM/FMIf you have the earlier setup you may have to make an adapter. The connectors for the speakers are available if you look for trailer connectors. I believe they used a square three pin, a square four-pin with one pin missing, and a flat three pin which will fit to a flat four-pin.
You may have to run separate ground wires for early two-wire systems, they shared a ground between speakers and many new radios aren't fond of that arrangement. You'll have to run grounds for the one-wire.
It's easy enough to fish one wire through the doors, and you can get different colors or mark the ends as to where they go. Just a basic 18 AWG wire like the factory used is enough unless you're spending big money on amplifiers.
http://www.engadget.com/2008/03/03/audiophiles-cant-tell-the-difference-between-monster-cable-and/Depending on the mood at AMC and the alignment of Mars and Jupiter there may be a intermediate connector that combines the two three-pin connectors into one six-pin connector, those as well are available as trailer connectors and you can just hack that up or you can sell someone restoring an old AMC that adapter.
Power connections for the early wires are just spade quick-connects together in a connector, the three-pin one is Orange-Illumination, Brown-Ignition, and Black-Ground. Sometimes there is no ground wire, usually with AM radios.
There is no constant battery power in the old shaft radios, so you'll have to fish a wire up to the solenoid; put a fuse in right there in case the wire rubs going through the firewall. Likewise, there is no blue wire for un-dimmed illumination and you'll have to tap into the wiring behind the light switch for the parking lights.
There is often a white wire included for a power antenna or the optional factory amplifier.