00PerfRedGT

i had a question about the factory head unit. i have installed several sound systems on mustangs and other cars and on my other mustang i put an aftermarket head unit in before i installed subs. with my new GT it has a factory haed unit in and i plan on keeping that way. how hard is it to hook up an aftermarket sound system to a factory head unit? is there anything that i need to know or need to get for installation? and also what about the RCA jacks? thanks

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2000 Boss Shinoda Lv.1 Performance Red w/ Tan Leather #00-01
Wynjammer Supercharger, SCT, MAC O/R H, MAC SS CB, B&M shifter, Hotchkis C/Cs, Tokico shocks & struts, Hotchkis 2" drop, Chrome FR500s 18x9 & 18x10, 255/40/18 & 285/40/18 Michelin Pilots, Smoked Headlights w/ 8000k, Smoked Foglights w/ 8000k, 5% Tint, Blackout tails, Honeycomb Panel, UPR Interior Billet, GT Hood & Side Scoops, Audiobahn system, Royal Purple Synthetics, Steeda Oil Seperator, Spec Stage 3, Spec Aluminum Flywheel