Hooking up system to a factory infinity sound system in a jeep grand cherokee.

Tap into a signal wire (rear speaker wire) using a line level converter.
These convert the High level speaker wire output into low level RCA output that is accepted by your amplifier.
Get a good one Pacific Accessory Corporation (PAC) sni-35 is the best IMO.
The TRUNK-LOC is the SAME as the SNI-35 - but it incorporates a voltage sensing input - so you dont need to run an additional turn-on wire. The voltage is sensed through the speaker wire - and it turns on the amplifier.

Then you will need a fused power wire from the battery,
Ground to chassis, and remote turn on lead (if you use a standard line level adapter).

An amplifier installation kit will give you everything you need to add 1 amplifier (except the converter)

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