(+) and (-) wire colors??

You could determine ground with either the resistance or continuity function of a multimeter. To check if a wire is a ground for example (using continuity function), clamp one lead to bare body metal or bolt, and touch the wire in question. If you get a beep, you've got a complete circuit, ie: the wire is ground, connecting to the car body.

Ditto to check for positive, clamp one lead to known 12 volt point (say the battery positive post, and touch the wire in question.

Instead of a multimeter, you can use a simple circuit tester, lights when there's continuity:

 

Most of the ground wires are white with black and some are brown. But you'll better check it!
And ground is (-) in Prius and nearly every other car.
For (+) what are you looking for? Permanently/Battery(+) or ACC(+) or Ignition(+) or only Brakelight(+)?
And where do you need it?
I could look in the wiring diagram.

 

In this case, I would definitely switch to "Voltage"!
Because you could "send" a current to the unknown wire only protected by the internal resistance of the Multimeter or maybe a fuse. (Not nice, if it melts and you don't have a new one.)

While thinking about: It's the same problem with the ground wires...

I would only use a Multimeter at "Resistance" or "Continuity" if there is no voltage in the system (battery disconnected). Call me wimpy, but there is so much electronics. It isn't one of the old days' car.

And with "Voltage" one can also identify +12V and GND, if one checks the difference(=voltage) between known and unknown contacts.

 

I was wondering about that. Maybe someone else can comment: is it ok to check for continuity involving a wire with 12 volt supply? Say with either multimeter in continuity or resistance scale, and/or a circuit tester probe.

 

The manual of my Multimeter says:
"
D Resistance measurement
️ Make sure that all the circuit elements, circuits, component parts and other measuring objects that you want to measure are absolutely idle.
...
E Diode test
Proceed as follows to measure diodes or semiconductor sections or to carry out continuity tests of idle lines and cables
...
️Make sure, when carrying out a diode test, that the diode and the circuit, into which it might have been mounted, are absolutely idle...
"
But I think this is only relevant for accuracy of measurement.

Referring to the specs:
In both modes the measuring voltage is up to 3 V DC.
Not much, but maybe too much for a sensitive electronic component?!

In resistance mode there is a protection resistor of 20 MOhm. So there could be a short circuit current of 12V/20MOhm=0,6 microA max. At least that shouldn't be any problem!

In diode test mode (for continuity tests) there is no protection! This could damage the Multimeter, I think.

In voltage mode this can be ignored
But I would like to learn better from a pro!

 

Yeah, a meter tests resistance/continuity by measuring what voltage of its own it needs to put on the circuit before a certain current flows. If there is already a voltage on the circuit, the best thing that happens is you get a wrong reading.

I would second MickyMatter's suggestion to use the wiring diagram. It sounds funny to say there are no details to be found when they're so easy to find.

-Chap