best method of splicing HiLo Converter

It all depends on the type of vehicle, and what type of amp you're adding.  If the vehicle does not have a factory external amp, behind the deck is the best place.  This allows you to run your shielded RCA's from the deck (for theoretically less noise) to your new amp's location.  You have to be careful, though.  For instance, some new GM's with the Bose system do not have varying outputs from the deck, rather the volume is controlled at the factory amps.  If you were to tap in behind the radio, your volume controls would not affect the output of the new amp.

If you're simply adding a sub amp, many just tap the rear speaker leads at the back of the vehicle, which works most of the time.  However, some vehicles do not send full range audio to all the speakers, and have a factory sub.  In this case, it's best to tap in there.  Also, some vehicles, like the new Impalas with the premium systems cut the bass to the speakers as the volume goes up (manufacturer's trying to prevent the speakers from blowing).

I'm doing a similar thing. I have to add an amp to a premium Bose system in a 2004 Avalanche. I've been told a variety of thing. PAC says I can take an SNI-35 and tap into the audio out from the back of the radio. I've also been told I can go into the center console and there will be a 4inch subwoofer that I can tap into. I've been told special interfaces that I have to use.

The sad thing is that no one knows the wiring colors on that vehicle. I have spent alot of time looking for the wiring diagram for the vehicle with no luck. If you know something, let me know, please!

You should be able to order the wiring diagram from Chevrolet.  It'll coast $40 or so.

As far as using t-taps or scotch lok connectors, I always recomend against those as they introduce noise.  Ues butt-splice crimp connectors or solder.

Asundj, if you don't want to cut the existing wires, strip, wrap, solder, and tape.  I agree with DYohn, that taps can introduce noise, and they can cut some strands of the wire, not just the insulation.  I do like crimp caps, though.

Fosgate 3, if you just need the wiring so you can add a sub, just trace the twisted pair that runs to the stock sub and tap there.

Wxxyz, I'd probably use line out converters and y-splitters, for 2 amps off the back speakers.