Here's the item everyone on the Eclipse board uses to hook into a factory stereo, it's a direct match if you are replacing your head unit altogether (hmm, looks like they are out): http://www.sounddomain.com/sku/PERMITAH

Here's the wirning harness adapter for your power connections: http://www.scosche.com/scosche/connectors/MI02.asp

I've never seen a 6-pin splitter, but you could search... I'd think with an Exacto knife and some solder you could spit things up yourself? You could connect both units to your OE power/ground, etc. cables. You could Y-adapt both units into your stock amplifier.

But you'd have to switch between the two sources - and to do this, you could tap into the turn-on lead for each head deck (the "switched power supply") and put it onto some kind of toggle switch. Up is RioCar, middle is off, and Down is stock head unit?

Just some ideas...