See my post about tap-ins. The car I recently bought has a stock amp that should work with empeg. I was going to use the tap-ins to put RCAs on the stock amp input wires without cutting-off the stock harness. Unfortunately, I haven't even started this project because I'm still wating for something to finally be released. The tap-ins may help your situation, but I'm having trouble figuring out what exactly you're trying to do with the stock deck. I probably just don't know MB's head units well enough.

As for soldering, I don't think you can beat Cold Heat for just soldering wire together. While I'm no soldering expert, I have been doing 2-3 audio installs per year for a while now and Cold Heat makes things so much easier/quicker. I wouldn't mind helping you with this project. Shoot me a PM if you would like me to come by...