My question is - is it possible to relocate the stock stereo and still use it for the dic while installing the new stereo into the dash?
I've relocated a GM stereo to the trunk in my 2002 Impala. (I mounted it atop the amp I had there.) I found a 7- or 8-lead cable at an electronics store here that had just enough wires in it to provide the minimum connectivity for getting it running. Basically, I could reset the oil life monitor and do other similar functions with it. But I had to be in the back seat or trunk to use it.
That said, rather than re-locating the radio to the trunk, I recommend you get a setup that replaces those features and keeps the DIC functions within easy reach of the driver.
As for relocating the radio, going from memory, in the Impala, there were 2 power wires, 1 gnd, 2 wires for the speaker (warning sounds) and 2 wires for the serial bus: 7 total. I found some terminals which slide over the pins on the back of the radio. I attached those to the cable, then applied shrink tubing over the terminals. With the terminals positioned on the pins, a couple zip ties were used to anchor the cable in such a way that there was no tension on the wires/terminals. To be clear, since there was no connector housing for the terminals, there was no way to secure them to the pins on the radio other than using the zip ties on the cable. It sounds kludgy, but worked very nicely.
For sure, I wouldn't waste any money on the Metra 70-2103T.
Looking at the Metra 99-3303, it's expensive, but it does appear to have everything you need. And it will save you lots of time spent researching and gathering all the pieces. The only caveat is that it will only work with single DIN radios, which rules out the DDX470.