There is a remote wire in the back that should turn on your factory amp, you can splice into this to turn on your aftermarket amp, but don't use this for your remote turn on in your new aftermarket radio. That remote turn on should be a fuse in the glovebox that you can go into. The grounding nut for the aftermarket amp is also back there in the middle inside against the rear bumper, to get power there should be a Power Distribution box in back of seat near spare tire area, you can use that as it is already fused, so no need for inline fusible link and no need to go to the battery post for power. Run your RCA's all the way up to your new headunit. One option is to Run Speaker wires from your amp the the speakers in your car (lot of work) or again find the OEM amp and locate the speaker wires going into the amp and tap there to get your signal, of course you should disconnect or bypass the factory amp. This saves you a lot of time running a lot of wires. Good Luck

__________________
2003 BMW E53 X5 4.4i - M62 engine (Non Sport with Premium and Cold Weather Package)
Color: Steel Gray Metallic
Mods:4.6is Exhaust (Pipes, Mufflers and chrome oval tips) BMW Sport package Taillamps, BMW Tire Valve Stem Caps
Medium tinted front windows
JVC HD Radio
Alpine 4x100 channel Amp
Orion 600W Mono HC Amp
Three 12" Sony Xplode in subwoofer enclosure