Connect 2 outputs to one speaker???

I have a simple question that's probably been asked and answered before (I tried searching but couldn't find it)

Can I connect two outputs on my carstereo to one speaker? For example in my 2-door (and 2-speaker) car, can I connect the front+ output AND the rear+ output to the speaker's + input (and same with the -) on each side/channel, to utilize my 4 channel stereo better?

And if so, what happens to the audio signal? In my mind it shouldn't affect the audio signal at all, but I could be wrong.
 
Simple answer. No 🙂

The reason is because any slight difference in audio content, be that stereo channel differences or level differences will cause one amp to force current into the other amps output causing overheating and damage. The amps themselves may be unstable or (depending on circuit topology such as being Class D), they may just literally destroy themselves instantly.
 
You can't join amplifier outputs.

If you need to mix programs , do that at line (preamp) signal level , search for mixer circuits, even passive ones, and then amplify the mixed signal with a single amp which will drive that single speaker.