Audio Systems: 1993 Mercury Gran Marquis Factory stereo replacement, gran marquis, ford wiring

Question
thank you.  I plan to continue to use the existing Ford amp, and will purchase the "integration harness."  However, is there any way to determine if I need to run the 4 additional pairs to the amp?
-------------------------

Followup To

Question -
In an attempt to have one of the chain electonic stores replace my factory stereo, I was informed that I would need special wiring performed (@ $120 price) because my 1993 Mercury Gran Marquis has a factory amplifier.  The car does have a factory amplifier, but can't I just remove the old unit and stick a new one in its place?

Answer -
Here is the lowdown on your premium amp problem;
http://www.installdr.com/Harnesses/Ford-Wiring.pdf

Now, if you want to remove and bypass the Ford amp, you can, but it can cost more than the shop quoted you to do so. Some of the Fords also require 4 pairs of speaker wires to be run from the new headunit to the amp location, and an integration harness is still a good idea, even when doing this.
One last thing, ask for the brand and model of the adaptor. It should be a very good one for that much money.

Answer
If you use the integration harness, you should not have to run new speaker wires, as long as the speaker ground wires are not tied together. Older Fords had them common grounded, and newer stereos cannot run that way. A multimeter is needed to test for this.