RADIO FOR 1994 Grand Cherokee

You can disconect the battery for 10 or so minutes, and try it again after reconnecting power. If this does not work, you likely have a wire problem, problems with the speakers themselves, or a problem with the radio head unit.
DO ALL OF THE FOLLOWING WITH THE NEGATIVE BATTERY CABLE DISCONNECTED! Leave it disconnected for at least 5 minutes before beginning work if you have ANY SRS devises (Airbags, pretensioners). You will likely have to remove dashboard panels to gain access to the radio head unit, and door pads or interior panels to access speakers.
The simplest way to check speakers is to pull the speaker out, and using an Ohmmeter to test the resistance of each speaker. On most mobile speakers, the resistance will be 4OHM. You should also hear a little pop or crackel sound when touching the probes to the speaker terminal. If all speakers test good, you need to get a wire diagram (or an aftermarket radio harness) and figure out what leads on the radio harness goes to each speaker. An aftermarket radio harness will plug in to your vehicle's harness and make testing a whole lot simpler, as every wire is marked as to it's function.
Using a paperclip or something similar, bridge the + and - for each speaker connection at the plug that goes into the radio (one at a time). Use the ohm tester, set to 1k ohm. It should read 0 or just a few OHM (IE.001 to.003 in the K ohm setting) at each plug that normally plugs into a speaker. If you get an infinity reading, a wire is broken.
Next test. If you have a good reading on this test, remove the paperclip and test again. This will test the wires for shorting aganst each other. The tester should read infinity.
Next find a good body ground. Test each speaker connection on the harness (red probe) to that body ground point (black probe). This test should read infinity for both wires at the speaker.
Repeat for all problem speakers. If wires all test good, and all speakers are in good condition, see a dealer, junkyard, or aftermarket supplier about getting a new radio.
If you find it is a wire problem, it's simple as running a new speaker wire to each of the mafunctioning speakers. I personally like the pull and tuck method, as you have to remove less interior panels to hide the new wires.