This depends on whether this "radio" is meant to drive speakers directly, or meant to drive a separate power amp that then drives the speakers.
The typical non-power audio signal for interfacing to other equipment is about 1 V RMS with 600 Ω impedance or less. That is sometimes called line audio or a line level signal. A separate stand-alone power amp would most likely expect such a signal as input.
If this radio drives speakers directly, then it is really a combination of a radio receiver, and power amp all in one. If the receiver and power amp were separate boxes, then you'd expect the signal between them to be line level. However, in this case that is totally internal to the box, so is probably not accessible, and may not even exist in that form internally anywhere. All that comes out are signals intended to be connected directly to the speakers.
Car speakers are usually 4 Ω. From that and knowing the output power you can compute the voltage. For example, let's say the radio can drive each of the speakers with up to 15 W of sustained power.
P = V²/R
When V is the EMF in volts, R the resistance in Ohms, then P is the power in Watts. Working this backwards to solve for V yields V = sqrt(15 W * 4 Ω) = 7.7 V RMS. For sine waves, the peaks are sqrt(2) higher, or 11 V. Peak to peak would therefore be 22 V.