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I think first of all, you would need a car battery that is charged... or a 12 V. DC power source, to power the car radios in question.
Then obviously, you would need a speaker pair (at least - switched between speaker outputs, if the stereo has quad speaker outputs) to test the speaker outputs, the general tone of the sound, etc. Note that impedance matching is often important (unless you have variable or adaptive impedance speaker outputs, of course, which increases the usable range) so the speaker's impedance should be matched to the rated output impedances of the stereos in question.
And, you'd need an antenna for some of them, for the radio functions. A car antenna would do, obviously.
For CD players, you need one unscratched and not-cracked audio CD, to test the CD playing functions.
And a voltmeter helps, when dealing with voltage measuring, whether on the power source, or speaker outputs (not that voltage measuring on them is often needed).