Early radios had only positive outputs to the speakers. The speaker negatives were all tied to ground. You cannot just connect all the radio negatives together on radios with a positive and negative for each speaker, it will damage the radio. The changeover occurred starting in the mid to late seventies. To put a newer radio in an older car requires un-grounding the speakers negatives and providing a speaker negative wire back to the radio. There used to be adapters which used transformers that had positives and negatives for each output from the radio and only a positive for each speaker and a single ground with no power. That type of adapter is what you would need.
Another thing that matters is impedance matching. Most car speakers were either 8 ohm or 4 ohm. If your speakers were 8 ohm then your radio or amplifier should be 8 ohm as well. For low power levels it is not critical for an exact match. But an exact match allows the most power at any volume to reach the speaker. In some cases of severely mismatched impedance, blown speakers or fried amplifiers are the result even if the sound was not loud, but the volume had to be turned way up to hear well.
There is a specific type of transformer called an impedance matching transformer. Let's say your radio is 4 ohm output but your speakers or booster amp/eq is 8 ohm input. you need a 4/8 ohm matching transformer able to handle the max wattage of the radio. Then if the booster/eq has 8 ohm, but your speakers matched your radio, then you need another set of 4/8 ohm transformers to match the speakers to the booster/eq and they have to be rated for the max wattage of the booster.
If the impedance matches, then all you need are 1:1 transformers rated for the max wattage to connect the booster/eq. that's what those adapters used. In no case do you tie any outputs together, it will likely damage the radio. If the radio has LF,RF,LR,RR outputs, then only hook up the LF and RF to the booster and leave the LR and RR disconnected and taped off.
On a side note, some amps and radios cheated on the impedance and designed 6 ohms outputs "close enough" to either 4 or 8 ohm to work. They also tended to boast ratings like 1,000 watts per channel yet cost less than half of a quality radio properly rated at only 100 watts per channel.
