Let me make sure I have this all straight. You have an aftermarket radio installed, along with speakers in the dash, and some speakers in the soundbar that do not work. You also have an aftermarket sub and amp, which is working. Is that correct?
Unless the amp you have is a 4 or 5 channel amp, it is likely only powering your subwoofer; the dash and soundbar speakers are being powered by the aftermarket radio's internal amp. In order to determine why your rear speakers are not working, you will need to remove the radio and start there. Once removed, you should look for RCA cables plugged into red and white RCA connectors on the radio. If your amp is only powering your sub, you will see only one set (red and white) of RCA cables there. If this is the case, the amp is not your problem, as the speakers are wired into the radio's amp.
Next, look at the wire harness coming off your radio. There should be lots of wires in this harness, you will be looking for 4 in particular. The rear speaker wire colors are purple and purple with a black stripe, along with green and green with a black stripe. Make sure there are wires connected to these, and that there is no exposed bare wire that could touch metal anywhere. Ensure the connections are tight, and that there are no breaks or loose connections with these wires.
If you have a multi-meter or an ohm meter, you can perform a simple test to ensure those speaker leads don't have a break in them between the radio and the speaker itself. Connect the speakers back into the soundbar, two wires for each speaker. Go back to the radio harness, and cut the purple & purple/black and green & green/black wires at the connection into the vehicle harness. Then, take the vehicle harness's side of those wires and test for resistance- there should be somewhere between 3 and 10 ohms of resistance when metering the speaker wires. If you get "OL" displayed on the meter, than you have bad wiring somewhere between the radio and the speakers. If you get a resistance reading within the range (3-10 ohms) you probably just have blown speakers.
My guess is that the soundbar speakers are not connected properly, or that the speakers are blown. If you do have bad wiring, just run new speaker wires from the purples and greens on your radio to the soundbar, and connect the speakers to that new wiring. Purple is the right lead, while green is the left.
Happy troubleshooting!