Ran into a Doosy, Can anyone help?

Here's what I have:

2002 Pontiac Grand Prix GTP, Factory head-unit, heads-up display, factory speakers and the kicker...... OnStar!

Here's what I want to do.....

I want to add a pre-amp EQ (PPI EQP), two amps, a sub and replace the factory speakers.  I want to keep the factory head unit (the head unit displays stereo information in the heads-up display..... way too cool an option to loose!).  I know this can be done using in-line converters.  This would all be a walk in the park if it had not been for the OnStar interface.  I have found interface kits for adding a new headunit and amp to an OnStar system but viewing the diagrams these won't work using the factory headunit.  So with all of your wisdom.....  can you give me some advice here?

I have never installed in a car equiped with OnStar... but since you want to keep the factory head unit, it would seem that the easiest way to do what you want is to use basic speaker-to-line level converters off the speaker outs of your head unit.  Run line level signals to your EQ, amps, etc.  This shuld allow all factory interfacing with the factory head unit to stay intact.

Am I missing something?

I think that the onstar system interfaces at two points, at the speaker and at the radio, if I interupt the signal to the speaker won't I loose the audio from the onstar?  I wish OnStar would provide diagrams but they don't.  It's risky at best!

vseven is right, it shouldn't matter as long as you have grabbed the speaker level signal at a point where it is heading straight for the speakers; ie: straight wire to the speakers.  I don't really think any level differential that he mentions should pose a problem, by the way, so I would think you can get by with two (one for frint and one for rear) converters.

Now, if this car has a factory "premium" audio system, especially if it has seperate tweeters in the dash or somewhere, you have more problems to worry about.

I highly recomend getting hold of the factory wiring diagrams for this car before you simply start swapping things.  Or bite the bullet and go let a pro shop do the conversion...

OK, I did a little research, and if you are keeping the factory head unit, all you need to do is use one set of speaker-to-line-level converters.  The OnStar interface is in the radio, upstream of the speaker wires in the wiring harness.  If you add aftermarket amps, speakers, etc, the OnStar signals will still work as they do stock with no level problems.

I recomend you get the Metra wiring harnesses that plug into yur head unit AND the one that plugs into your factory wiring harness.  Us eboth so you can "splice" into the factory wiring without damaging it.  Use the Metra wires for your connections.  The Metra part numbers should be 70-1858 and 71-1858.

If you are adding an aftermarket head unit, you can still keep the OnStar functionality with a specially designed adapter.  They run about $100 and plug into the Molex connectors of the factory wiring harness.  SoundDomain.com sells one.

Cheers.

I REALLY appreciate your help on this guys!  So which speakers do I attach the line converter to? Front or back?  Would I be able to do both to get front/rear control as well? Since I'm using a pre-amp EQ that takes F/R inputs.

Take the speaker signals for the factory front L-R to one line converter and run RCA's to your EQ for the Front signal.  Take the speaker level signal for the factory rear L-R to a second line converter, and run those RCA's to the EQ for the Rear signal.  Then, you will need to connect new front AND rear amps to the outputs from the EQ, and run BOTH front and rear speaker wires (new wires) from your amps back under the dash to connect to the factory wiring harness that goes out to your speakers.

What you will have done is simply interupted the speaker output from your head unit and sent it to your after market EQ and amps, then back to the factory wiring headed to the speakers.

Does this make sense to you?