aftermarket head units and OEM amplifiers

I'm sure other installers have encountered many 1990s imports and domestic luxury vehicles with "premium sound" OEM stereo systems.  Many include 4 or more midrange drivers, tweeters, and even subwoofers.  The systems are powered by an OEM amplifier that [i]mysteriously[/i] receives a high level signal of some sort from the OEM headunit.

I've not worked with any of the more expensive harnesses, but i'm wondering if there is a way to send a comparable signal from an aftermarket Head Unit to the OEM amplifier. 

I was looking to do this specifically in a 1995 Cadillac Deville Concours, but I was unable to identify common wires at the OEM amp's harness and OEM HU harness.

From my understanding of amplified stereo systems, there is a HU that provides a signal.  this signal is communicated to the amplifier traditionally by RCA patch cables.  These patch cables may be a matter of convenience for aftermarket HUs.  But they still use a +/- signal just as high level speaker outputs from a deck.  So even in the case of OEM amplifiers, ther's still gotta be a +/- signal connection for a front left channel, front right channel, rear left channel, and rear right channel.  Whether or not that signal is high level or low level is the question.  Even though it's not using the aftermarket convenience of RCA patch cables, it's still communicating a signal from 4 channels to the amplifier.

The question is how are the OEM HUs communicating that signal (apparently, speaker wire-type low level outputs).  And how can the 4 channel speaker outputs (or possible low level outputs) be modified to communicate a comparable signal to the OEM amplifier?

attach patch cables to aftermarket HU's low level outputs, then splice the inexpensive patch cables to get a speaker output-type low level signal?  but then, it's still gotta go into the right input on the amp?  how would one identify that?  do the more expensive wiring harnesses do all of the above for you?  if so, then I'm still interested b/c the vehicle in question at the moment is a Deville Concours for which no Metra or American International install support is available.

Your supposition is correct... in general.  Many OEM "premium" systems use line-level signals that are passed through the car's wiring harnesses.  Most do not use "RCA" cables.  Often they use common ground systems with three wires (or two wires and a shield) carrying the signal.  Some systems do use high level signals but usually it is cheaper and easier for the manufacturer to use line level and external amps.  There are aftermarket adapters available for some but not all cars.  Each car is different.  You will need the factory electrical wiring diagram for the specific car you are working on if you want to try and keep any part of the OEM system.

The reason most installers simply bypass or remove all OEM systems is that even the so-called premium or audiophile systems are actually one step above crap.  Car manufacturers must use the lightest, cheapest components possible in order to both keep the manufacturing cost down and to keep the weight down for CAFE reasons.  There are only a few OEM systems that are worth keeping, IMO: The Lincoln THX certified system (the first auto manufacturer to ever acheive THX certification) and the Acura DTS 5.1 system.  Even the Mark Levinson-branded system in some Lexus cars leaves a lot to be desired..

As far as your 1995 Caddy is concerned, I would guess the OEM system pales in comparison to most mid-level after market equipment available today.

Good luck!

looks like https://www.logjamelectronics.com/intfaceconv.html has "head unit replacement converters" (for a hefty price).  and it looks like the HRC-GM32 is specified for the 94 - 98 Devilles w/Bose system.

I don't recall seeing any "BOSE" emblems on the OEM equipment, though.