The front speakers are really simple to get too. I'd check there first to make sure they are actually connected. Then pull out the radio and check the wiring harness. If it's an aftermarket radio, they either did it correctly and used a harness, or who knows what they did and could have just started splicing wires together with electrical tape. Maybe the speaker wires came loose? The radio also isn't hard to pull out to inspect.
Some newer radios have a feature that disables the internal amp and only sends signal through the pre-amp RCA outputs. If wires are connected with no shorts, I would check that. What is the model of the Kenwood radio?
So it sounds like the original owner wired up the radio to go through an amp to the speakers, but that doesn't mean that you couldn't just rewire the head unit to use the speaker wires. If it doesn't have the proper wiring harness adapter, you can get one of those pretty cheap. An amp will probably sound better, but in general Kenwood head units sound decent with most putting out 22 watts RMS these days.
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