Bypassing the factory Amp in a '91

Background, I have a 91 Mustang, with the factory 6-Speaker setup (2 in dash corners, 2 in the door, 2 in the rear seat area).

The old Sony receiver I had was recently stolen. Its not all bad, it had intermittent sound to some of the speakers and featured a cassette deck!

I purchased an Alpine 9857 deck, and I assumed (bad me) that it would be plug and play just as the Sony was. No dice. Inspecting the harnesses in the radio area, I noticed that the factory feeds to the amp use a single common ground. This evidently won't work with the new deck as I plugged it in and no output (I tried CD and radio to be sure it wasn't one or the other).

Is there an easy (I am lazy) way to get this working without resorting to new wires to all the speakers, and/or installing an amp? I am really against installing an amp, but if push comes to shove I guess I will do it.

Any shortcuts you guys can think of will be greatly appreciated.

Thanks

Thanks I may try that, but the more I think about it, the more I think I will just rewire the entire thing. I have to pull it all apart to replace the "high-end" paper cone speakers anyhow.