new stereo/ old vehicle

Run a wire from your battery, fused that's the B+ (yellow) be sure it's from the positive terminal.

Ground the HU to the firewall or floor, whichever is closer.

turn the car on, test for voltages on the wires, there should be at least two, use the one that comes on with the key (ACC) red wire, tape the others up, you won't need them.

Now, you have all wires with no power on them, if you have the patience, check each pair and find your speakers. (don't short the wires on th HU)

you can also run a new pair to all four speakers, if you have that one speaker in the dash....unplug it, it won't work with your new radio.

After this, you're all done, don't forget the antenna cable..

Those are my suggestions, do a search and find out what wiring to use and how to make proper grounds. Should be an easy install.

be certain that there are no ground wires or postive wires in your way if you do the 'speaker test'

you can find the speaker pairs by testing resistance through the pairs, should be anything like 3.2-7.4 ohms...just something. This may save you a little time, put your live speaker outputs on it to see where it is, if it's the corresponding leads, connect them and use another pair to keep testing the speaker wires.

I'm willing to bet you will need new speakers too, that's an old car.