An iphone/ipod has a weak output period. The headset out isn't particularly clean and is intended for an earbud or headset with a much lower impedance than a head unit or EQ RCA or 3.5mm input. The preout coming from the multipin port is much cleaner but still very low in voltage. Then you have the loss from the Bluetooth module as decribed by Mike.
Try a JL Audio 2-channel line driver between the Bluetooth module output and the HU Aux input. It has a gain control that you can place anywhere without extending the signal path.

David