Kenwood KDC-MP8017

Try using the preouts for your test bench's inputs and see if you get sound then.  If so, then either the internal amp is busted, or the onboard amp is disabled (some head units have this kind of option in their settings menus).

Kenwood's support site lists, among others, Pacific Coast Parts as a distributor of replacement parts for their products.  If you go to their site you should be able to enter in your head unit's model number and see a listing of available parts.

Can you hear ANYTHING when the preamp outputs are connected?  I know you said that with the speaker outputs connected, you could just barely hear a tiny bit if you put your ear up to the speaker.

With only the preouts feeding your testbench an input signal, I'd expect there to either be full/normal output, or absolutely none at all.  It's certainly possible that both the preouts and internal amp are bad, but usually it'd be one or the other.  If both those components are bad, I doubt you'd be able to get anyone to buy it.

so you're saying you had nothing when you hooked the pre-outs to an outboard amp?? then it wont be the built in O/P IC... sounds like it's latched in a mute mode.... or possibly the EVC ( Electronic Volume Control) IC

are you sure about the pre-outs not working either??

lastly, meter the remote out lead for voltage... what do you have?   sometimes these units will go mute when you smoke the remote turn-on transistor.....