Best way to install a new head unit?

[TDK]

Well,

My son who is now driving our '94 DeVille Concours bought a Clarion DB365 USB head unit (with filler plates), to replace the non-Bose original unit, and so he could RCA-input his Sirius portable unit at the new head unit. Note that the original unit's CD-player was not ejecting propelry and I removed it to attempt refurbishment of the rollers that move the CD, (so the new unit is justified in numerous ways) .

The Clarion unit is a 200-watt by 4-channel unit with (4) RCA outputs. Http://www.clarion.com/us/en/products/2006...4643389085.html.

I am not at the car to "assist" yet and realize these are relativley complex questions to ask, but:

1. Is it possible to plug in the new head unit's (4) RCA outputs into the Caddy amp in the trunk and still use the original Caddy speakers? How are the speakers controlled then - how ever many there are?

2. Or is it simpler to wire directly to the speakers? I think the impedence-matching will be a big issue and wires in general will be a mess if all the speakers are to be wired up properly. A variation may be to replace (4) of the main speakers, but that still sounds like a mess.

3. Or, I told him - consider returning the Clarion head unit and use long RCA wires from the Sirius unit to the Caddy's amp in the trunk.

4. Or, are we completely out of our league here (as in don't have a clue how it is laid out to begin with) and we should just take the car to a car audio shop?

A link to a wiring diagram would be great.

Thanks in advance for any assistance.

Oh BTW, he really likes the Caddy as a daily driver compared to his Mitsubishi EVO that is tricked out 400+ hp, with all the bolts, bad-boxes and leds for SCCA racing. So I am hoping we can get past this hurdle with your help and continue with Caddy appreciation by his friends.

Thanks again.

UPDATE!

micahdaniel had posted an exact answer to my question - a few hours before I asked it! See his post regarding installing a radio in in a '95 Deville. Sheesh! what a great site - that knows what its readers need before they even ask.

Thanks and I'm still trying to figure out how that happened...