At least in 67 the speaker should be 8 ohm
I read on a retro radio site (not the company that retrofits old radios) that using a 4 ohm speaker on a radio designed for 8 ohm will harm the output transistor / amplifier.
Classic Car Speakers
"Note: Many replacement speakers now sold by the discount stores, auto parts stores and mail order restoration parts houses are labeled as "4 to 8 ohm compatible" and are in fact 4 ohm speakers. These speakers can damage the older transistor radios. 4 ohm speakers will measure around 3.2 ohms with a DC ohmmeter. An 8 - 10 ohm speaker will measure around 7.4 ohms with a DC ohmmeter. The DC resistance is the critical number for the Delco radios as the output transistor is in series with the speaker. A properly operating Delco radio should have 1.5 volts DC across the speaker voice coil. A 4 ohm speaker will draw twice the current of an 8 ohm and cause the amplifier transistor (the half dollar size device mounted to the black finned heat sink on the back of a Delco radio) to run very hot. The speaker cone should be displaced in an upward (away from the magnet) direction about 1/8" when a Delco radio is turned on. If the cone moves down toward the magnet, the speaker leads need to be reversed. Ford and Chrysler radios of the period used the same type of germanium output transistors as the Delco radios and also require 8 ohm speakers. However, Ford and Chrysler radios ran with 0 to 1 volt DC offset on the speaker depending on the radio model and vintage. Many will not deflect the cone when turned on like the Delco radios."
I can not speak to the OEM wiring and a fader. But it would seem that 2 x 8 ohm speakers in series (rear) would equate to 16 ohms. If they are in parallel they will be 4 ohms.
With an 8ohm front speaker and the rears are in parallel (4 ohms) the net would be some where between 8 ohms (all the way to the front) 12 ohms halfway between front and rear and 4 ohms all the way to the rear.
On the other hand if the rears we're in series, (16 ohms) front (8 ohms) the result would be 8 ohms all the way to the front. 24 ohms half way between front and rear and 16 ohms all the way to the rear.
I'm no speaker pro. And we are talking impedance rather than true resistance. But I think my logic applies
As for wiring
Radio positive to center of fader, one side to front speaker the other to rear speakers whether the rears are in parallel or series is how you wire them. A common ground between all the speakers and the radio negative lead.
Post a photo of the back of the fader
